:root{--ink:#202326;--muted:#61666c;--line:#e5e7ea;--soft:#f5f6f8;--blue:#245de1;--blue-soft:#eef3ff;--red:#b93e4f;--red-soft:#fff0f2;--green:#147a59;--green-soft:#ecf8f2;--accent:var(--blue);--tint:var(--blue-soft);font-family:Arial,Helvetica,sans-serif;color:var(--ink);background:#fff;font-synthesis:none}
*{box-sizing:border-box}body{margin:0}button,input,a{-webkit-tap-highlight-color:transparent}button,input{font:inherit}button,a{touch-action:manipulation}a{color:inherit;text-decoration:none}button{cursor:pointer}button:disabled{cursor:default;opacity:.45}[hidden]{display:none!important}:focus-visible{outline:3px solid var(--accent);outline-offset:5px}.skip{position:fixed;top:10px;left:12px;z-index:10;background:#fff;border:1px solid var(--ink);padding:12px;transform:translateY(-200%)}.skip:focus{transform:none}.header{max-width:1280px;margin:auto;padding:22px 40px;display:flex;justify-content:space-between;align-items:center;border-bottom:1px solid var(--line);gap:20px}.brand{font-size:15px;font-weight:600;display:flex;align-items:center;gap:10px}.brand img{object-fit:contain}.quiet-link{color:var(--muted);font-size:14px;min-height:44px;display:inline-flex;align-items:center}.quiet-link:hover{color:var(--ink)}main:focus{outline:none}.welcome{min-height:100svh;display:flex;align-items:center;justify-content:center;flex-direction:column;gap:36px;padding:40px 24px}.welcome-sprite{width:144px;height:216px;object-fit:contain}.start{display:flex;align-items:center;justify-content:center;min-height:48px;min-width:144px;padding:12px 30px;background:var(--ink);color:#fff;border-radius:6px;font-size:16px;transition:background .15s,transform .15s}.start:hover{background:#3d4247;transform:translateY(-2px)}.selection{max-width:960px;margin:0 auto;padding:72px 40px 80px}.selection h1{font-weight:500;font-size:32px;line-height:1.25;margin:0 0 42px}.subject-list{display:grid;grid-template-columns:repeat(3,minmax(0,1fr));gap:30px}.subject{padding:24px 0 22px;border-top:2px solid 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280px;gap:42px;align-items:start}.scene{min-width:0}.scene-top{display:flex;justify-content:space-between;align-items:center;gap:16px;min-height:56px;margin-bottom:12px;flex-wrap:wrap}.equation{font-size:21px;max-width:100%;overflow-x:auto;overflow-y:hidden;padding:5px 1px}.equation .katex{font-size:1em}.segments{display:flex;gap:3px;padding:3px;border:1px solid var(--line);border-radius:6px}.segments button{font-size:14px;background:none;border:0;padding:8px 11px;min-height:36px;border-radius:3px;color:var(--muted)}.segments button[aria-pressed=true]{background:var(--soft);color:var(--ink)}.plot{width:100%;height:auto;aspect-ratio:800/520;display:block;overflow:hidden;border-radius:6px;background:#fafbfd;touch-action:none}.plot text{font-family:Arial,Helvetica,sans-serif;font-size:15px;fill:#747b85}.plot .axis-label{font-size:18px;fill:var(--ink)}.plot .handle{cursor:grab}.plot .handle:active{cursor:grabbing}.plot .handle:focus{outline:none;stroke:var(--ink);stroke-width:4}.plot 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0}.angle-controls{padding-top:10px}.pack-row{display:grid;grid-template-columns:1fr 1fr;gap:16px}.pack-row .control{margin-bottom:20px}.pack-title{margin:0 0 20px;font-size:16px;font-weight:500;display:flex;gap:8px;align-items:center}.pack-title:before{content:'';width:10px;height:10px;background:var(--green);border-radius:2px}.pack-title.second:before{background:var(--blue)}.pack-group+.pack-group{border-top:1px solid var(--line);padding-top:22px;margin-top:8px}.best-chart{padding:25px 26px 32px;background:var(--soft);border-radius:6px;min-height:360px;display:flex;flex-direction:column;justify-content:center;gap:30px}.bar-label{display:flex;justify-content:space-between;gap:16px;align-items:center;font-size:16px;margin-bottom:11px;flex-wrap:wrap}.bar-label .katex{font-size:1.05em}.bar-track{width:100%;height:50px;background:#e8ebee;border-radius:3px;overflow:hidden}.unit-bar{background:var(--green);height:100%;width:50%;transition:width .12s}.unit-bar.second{background:var(--blue)}.best-result{font-size:18px;margin:0;padding-top:20px;border-top:1px solid #d6dadd;line-height:1.5}.best-result strong{font-weight:500}.comparison-label{color:var(--muted);font-size:14px;margin:0 0 20px}.exchange-plot{background:#fafbfd}.exchange-result{display:flex;align-items:center;gap:20px;flex-wrap:wrap;padding:16px 0;font-size:23px}.exchange-result .katex{font-size:1em}.exchange-result .separator{width:18px;height:1px;background:var(--muted)}.error{color:var(--red);font-size:14px;line-height:1.5;min-height:22px;margin:12px 0 0}.noscript{position:fixed;bottom:20px;width:100%;text-align:center;font-size:16px}.sr-only{position:absolute;width:1px;height:1px;overflow:hidden;clip-path:inset(50%);white-space:nowrap}.katex-display{margin:0}.empty-message{padding:40px 24px;text-align:center}
.selection.geometry{--accent:var(--red);--tint:var(--red-soft)}.selection.ratio{--accent:var(--green);--tint:var(--green-soft)}
.welcome-sprite,.brand img{image-rendering:pixelated}
.scene{container-type:inline-size}
@container(max-width:480px){.plot text{font-size:28px}.plot .axis-label{font-size:32px}}
@container(max-width:350px){.plot text{font-size:32px}.plot .axis-label{font-size:36px}}
@media(max-width:960px){.workspace{grid-template-columns:minmax(0,1fr) 245px;gap:28px}.controls{padding-left:22px}.header{padding:18px 28px}.lab{padding:28px}.scene-top{align-items:flex-start}.equation{font-size:19px}.subject-list{gap:22px}}
@media(max-width:720px){.header{padding:12px 20px}.brand{font-size:14px}.lab{padding:26px 20px 36px}.lab h1{font-size:25px}.workspace{grid-template-columns:minmax(0,1fr);gap:24px}.controls{border-left:0;border-top:1px solid var(--line);padding:24px 0 0;display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:0 24px}.controls>.control:nth-child(3){grid-column:1/-1}.checks,.control-actions,.controls>.note,.angle-controls>.button,.controls>.error{grid-column:1/-1}.control{margin-bottom:20px}.control-actions{margin-top:18px}.investigation{gap:12px;margin-top:24px}.investigation p{font-size:15px}.investigation .button{padding:8px 11px}.selection{padding:46px 24px 60px}.selection h1{font-size:28px;margin-bottom:32px}.subject-list{grid-template-columns:1fr;gap:14px}.subject{display:grid;grid-template-columns:94px minmax(0,1fr);gap:18px;padding:20px 0;align-items:center}.subject .topic-art{grid-row:1/3;height:94px}.subject h2{font-size:21px;align-self:end}.subject small{align-self:start}.topic-art svg{height:80px}.subject .topic-art .katex{font-size:.85em}.lesson{grid-template-columns:minmax(0,1fr) 110px;gap:16px}.lesson h2{font-size:21px}.lesson p{font-size:15px}.lesson .topic-art{height:96px}.lesson .topic-art .katex{font-size:.85em}.pack-group{min-width:0}.pack-group+.pack-group{border-top:0;padding-top:0;margin-top:0}.pack-row{grid-template-columns:1fr;gap:0}.facts{gap:12px 18px;font-size:14px}.facts .fact small{font-size:13px}.plot{aspect-ratio:800/520}.best-chart{min-height:300px;padding:24px 20px}.scene-top{gap:10px}.exchange-result{font-size:21px}.welcome{gap:32px}.welcome-sprite{width:128px;height:192px}}
@media(max-width:380px){.brand span{font-size:13px}.header{padding-inline:16px}.lab{padding-inline:16px}.selection{padding-inline:20px}.controls{gap:0 18px}.segments button{padding-inline:8px}.investigation{align-items:flex-start}.subject{grid-template-columns:80px minmax(0,1fr)}.subject .topic-art{height:80px}.subject h2{font-size:19px}.lesson{grid-template-columns:minmax(0,1fr) 88px}}
@media(prefers-reduced-motion:reduce){*,*:before,*:after{scroll-behavior:auto!important;transition:none!important;animation:none!important}}
@media(forced-colors:active){.start,.unit-bar,.topic-art,.bar-track{border:1px solid CanvasText}.plot .handle{fill:Highlight}.subject{border-color:CanvasText}}
/* The new models share the same quiet working surface. */
.facts .fact{max-width:100%;overflow-x:auto;padding-block:2px}.facts .fact small{flex-shrink:0}
.subject-list{grid-template-columns:repeat(4,minmax(0,1fr))}.subject.number,.selection.number,.lab.number{--accent:#6c45b0;--tint:#f3effb}.subject.number{border-color:var(--accent)}
.control-help{display:block;font-size:14px;line-height:1.5;color:var(--muted);margin-top:8px}.model-choices{width:max-content;max-width:100%;margin-bottom:26px;flex-wrap:wrap}.shape-choices{margin-bottom:22px;flex-wrap:wrap}.shape-choices button{flex-grow:1}.solid-lengths{grid-column:1/-1}
.pixel-sprite,.currency-sprite.euro{background-image:url(upgrade-sprites.png);background-size:300% 100%;background-repeat:no-repeat;image-rendering:pixelated;width:48px;height:48px}.pixel-sprite.ball{background-position:50% 0}.pixel-sprite.coaster{background-position:100% 0}.currency-sprite.euro{background-position:0 0}
.venn-board{position:relative;min-height:390px;background:#fafbfd;border-radius:6px}.venn-outline{position:absolute;inset:0;width:100%;height:100%}.venn-zones{position:relative;display:grid;grid-template-columns:repeat(3,minmax(0,1fr));gap:10px;padding:85px 34px 48px;min-height:390px}.tile-zone h2{font-size:14px;font-weight:400;text-align:center;margin:0 0 18px;color:var(--muted)}.prime-tiles{display:flex;gap:7px;flex-wrap:wrap;justify-content:center;align-content:flex-start}.shared-zone{padding-top:30px}.prime-tile{width:44px;min-height:44px;padding:5px;border:1px solid #bbc9e7;background:#fff;border-radius:4px;font-size:16px}.prime-tile.right{border-color:#b7d9c8}.prime-tile.shared{border-color:#9174c0;background:#f3effb}.prime-tile[aria-pressed=true]{outline:3px solid var(--accent);outline-offset:1px}.prime-tile .katex{font-size:1em}
.venn-zones{grid-template-columns:1fr 1fr 1fr}.venn-zones>.tile-zone:first-child{padding-left:12%}.venn-zones>.tile-zone:last-child{padding-right:12%}.venn-zones .prime-tiles{max-width:145px;margin:auto}.venn-zones .shared-zone{padding-top:0}
@media(max-width:960px){.subject-list{grid-template-columns:repeat(2,minmax(0,1fr))}}
@media(max-width:720px){.subject-list{grid-template-columns:1fr}.solid-lengths{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:0 24px}.solid-lengths>.control:nth-child(3){grid-column:1/-1}.venn-zones{padding-inline:9px;gap:6px;min-height:420px}.venn-board{min-height:420px}.venn-outline{min-height:420px}.tile-zone h2{font-size:14px}.model-choices button{min-height:44px}}
.selection.algebra{--accent:var(--blue);--tint:var(--blue-soft)}
.lesson .topic-art{padding:14px;overflow:hidden}.lesson .topic-art .katex{font-size:1.05em}.lessons .lesson{padding-block:23px}.lessons .lesson h2{font-size:22px}
.diagram{background:#f8fafc}.svg-math{height:100%;width:100%;display:flex;align-items:center;justify-content:center;color:var(--ink);font-size:18px;text-align:center;line-height:1.2}.svg-math .katex{font-size:1em}.svg-math.tick{font-size:14px}
.ratio-tabs{margin-top:20px}.controls>.segments{width:max-content;max-width:100%;flex-wrap:wrap}.controls .segments button{min-height:40px}.controls .control-actions{grid-column:1/-1}
.currency-sprite{display:inline-block;flex-shrink:0;image-rendering:pixelated;background-image:url(currency-sprites.png);background-repeat:no-repeat;width:40px;height:40px}.currency-sprite.pound{background-size:130.85px 58.16px;background-position:-13.85px -9.46px}.currency-sprite.dollar{width:58px;background-size:137.05px 60.91px;background-position:-72.57px -11.80px}
.exchange-result{gap:12px}.exchange-result .separator{margin-inline:8px}
@media(max-width:720px){.lesson .topic-art .katex{font-size:.85em}.lessons .lesson h2{font-size:21px}.svg-math{font-size:22px}.svg-math.tick{font-size:19px}.controls>.segments{grid-column:1/-1}.currency-sprite{transform:scale(.85);transform-origin:center}.exchange-result{gap:6px;font-size:19px}.exchange-result .separator{margin-inline:3px}}

@container(max-width:480px){.venn-zones>.tile-zone:first-child{padding-left:8%}.venn-zones>.tile-zone:last-child{padding-right:8%}.venn-board:has(.prime-tile:nth-child(n+5)),.venn-board:has(.prime-tile:nth-child(n+5)) .venn-zones{min-height:650px}.venn-board:has(.prime-tile:nth-child(n+5)) .venn-zones{padding-top:160px;padding-bottom:140px}}
.subject-list{grid-template-columns:repeat(3,minmax(0,1fr))}
.subject.statistics,.selection.statistics,.lab.statistics{--accent:#116b85;--tint:#edf7fa}.subject.statistics{border-color:var(--accent)}
.more-choices{flex-wrap:wrap;width:max-content;max-width:100%;margin-bottom:22px}.more-choices button{min-height:42px}
.more-handle{cursor:grab}.more-handle:focus{outline:none;stroke:#202326;stroke-width:4}.more-plot [data-more-drag]{cursor:grab}
.maths-panel{margin-top:15px;padding:18px;border:1px solid var(--line);border-radius:5px;line-height:1.55;overflow-x:auto;font-size:16px}.maths-panel p{margin:12px 0 0}
.coin-result{display:flex;align-items:center;gap:20px;margin:0 0 24px;min-height:90px;font-size:18px}
.toss-sprite{display:inline-block;flex-shrink:0;width:84px;height:84px;background-image:url(coin-sprites.png);background-repeat:no-repeat;background-size:200% 100%;image-rendering:pixelated}
.toss-sprite.heads{background-position:0 0}.toss-sprite.tails{background-position:100% 0}
.quad-checks{display:grid;grid-template-columns:1fr 1fr;gap:12px}.quad-coordinate{font-size:16px;background:rgba(250,251,253,.88);border-radius:3px}
.trace-control{margin-top:24px}.more-controls>.control-actions{margin-top:6px;margin-bottom:20px}.more-controls>.maths-panel,.more-controls>#coin-formal-controls{grid-column:1/-1}
@media(max-width:960px){.subject-list{grid-template-columns:repeat(2,minmax(0,1fr))}}
@media(max-width:720px){.subject-list{grid-template-columns:1fr}.more-choices{gap:3px}.more-choices button{font-size:14px;min-height:44px}.quad-checks,.trace-control{grid-column:1/-1}.quad-coordinate{font-size:23px}.coin-result{gap:12px;flex-wrap:wrap}.maths-panel{padding:14px;font-size:15px}}