@import url('https://fonts.googleapis.com/css2?family=DM+Sans:wght@400;500;600;700&family=Space+Grotesk:wght@500;600;700&display=swap');
:root{--ink:#16223a;--muted:#54637b;--blue:#2257db;--line:#dbe3ef;--orange:#e98732;--paper:#f4f7fc}*{box-sizing:border-box}body{margin:0;background:var(--paper);color:var(--ink);font:16px/1.6 'DM Sans',system-ui,sans-serif}button,input,select{font:inherit}header{max-width:1200px;margin:auto;padding:24px 32px;display:flex;justify-content:space-between;align-items:center;border-bottom:1px solid var(--line)}.brand{color:var(--ink);text-decoration:none;font:bold 27px 'Space Grotesk',sans-serif;display:flex;align-items:center;gap:12px}.brand span{font-size:16px;letter-spacing:.08em}.grade{font-size:13px;font-weight:700;letter-spacing:.13em;background:#dfe8fb;padding:6px 12px;border-radius:7px}main{max-width:1150px;margin:auto;padding:20px 30px 45px}.intro{padding:18px 0 23px}.eyebrow{font-size:12px;letter-spacing:.15em;font-weight:700;color:var(--blue)}h1,h2,h3{font-family:'Space Grotesk',system-ui,sans-serif;line-height:1.18}h1{font-size:clamp(32px,5vw,52px);letter-spacing:-.045em;margin:8px 0 10px}h2{font-size:27px;margin:0 0 6px}h3{font-size:21px;margin:8px 0 13px}p{margin:8px 0;color:var(--muted)}nav{display:flex;gap:8px;border-bottom:1px solid var(--line);padding-bottom:16px;margin-bottom:30px}button{cursor:pointer;border:0;border-radius:9px;background:var(--blue);color:white;padding:11px 17px;font-weight:700;min-height:44px}button:hover{filter:brightness(.94)}button:focus-visible,a:focus-visible,select:focus-visible,input:focus-visible,summary:focus-visible{outline:3px solid #edac35;outline-offset:3px}nav button{background:transparent;color:var(--muted)}nav button.active{background:var(--ink);color:white}.secondary{background:white;border:1px solid var(--line);color:var(--ink)}.section-top{display:flex;align-items:center;justify-content:space-between;gap:20px;margin:0 0 23px}.select-label{display:flex;align-items:center;gap:15px;margin:16px 0 22px;font-weight:700}select{background:white;border:1px solid var(--line);border-radius:8px;padding:10px;color:var(--ink);max-width:100%}.page-title{font-size:18px;margin:28px 0 12px;color:var(--blue)}details{background:white;border:1px solid var(--line);border-radius:12px;margin:10px 0;overflow:hidden}summary{cursor:pointer;padding:19px 21px;font-weight:600;list-style:none;display:flex;align-items:start;gap:15px}summary:before{content:'+';font-size:23px;color:var(--blue);line-height:1.1}details[open] summary:before{content:'−'}.problem-label{display:block;font-size:12px;letter-spacing:.08em;color:var(--muted);font-weight:700;margin-bottom:5px}.worked{padding:0 25px 24px 55px}.result{border-left:4px solid var(--blue);background:#eff4ff;padding:12px 16px;color:var(--ink);font-weight:700;border-radius:0 8px 8px 0;white-space:pre-wrap}ol{padding-left:23px}li{padding:5px 0}.lesson-grid{display:grid;grid-template-columns:repeat(2,1fr);gap:18px}.lesson{border:1px solid var(--line);border-top:4px solid var(--blue);background:white;padding:24px;border-radius:10px}.lesson .number{font-family:'Space Grotesk';color:var(--orange);font-size:27px}.equation{font-family:'Space Grotesk';font-size:21px;color:var(--ink);font-weight:600;white-space:pre-wrap}.tip{font-size:14px;background:#fff4e7;color:#6e441c;padding:12px;border-radius:8px;margin-top:15px}.visual{display:grid;grid-template-columns:1fr 1fr;gap:25px;background:#e7eefc;border:1px solid #cfdbf0;margin-top:24px;padding:25px;border-radius:14px}.visual input{display:block;width:100%;margin-top:14px;accent-color:var(--blue)}.bar-row{display:grid;grid-template-columns:repeat(18,1fr);gap:3px;margin:10px 0}.unit{height:35px;border-radius:3px;background:var(--blue);transition:height .2s}.unit.orange{background:var(--orange)}.bars{align-self:center}.bar-label{font-weight:700;font-size:14px;color:var(--ink)}.score{padding:9px 15px;background:#dfe8fb;border-radius:8px;font-weight:700;white-space:nowrap}.quiz{background:white;border:1px solid var(--line);border-radius:16px;padding:30px;max-width:850px;margin:auto}.quiz h3{font-size:25px;line-height:1.45}.topic-chip{font-size:12px;letter-spacing:.1em;color:var(--blue);font-weight:700}.answer-form{display:flex;gap:10px;margin-top:22px;align-items:end;flex-wrap:wrap}.answer-form label{flex:1;min-width:180px;font-weight:700}.answer-form input{display:block;padding:12px;border:2px solid #a9bad7;border-radius:8px;width:100%;margin-top:7px;font-size:19px}.format{font-size:14px;margin-top:8px}.feedback{margin-top:18px;padding:12px 16px;border-radius:9px;background:#eff3f9;color:var(--ink)}.feedback:empty{display:none}.feedback.good{background:#e4f5ec;color:#19613b}.feedback.bad{background:#fff2df;color:#805119}.tools{display:flex;gap:10px;flex-wrap:wrap;margin-top:18px}.hint{border-left:3px solid var(--orange);padding:8px 15px;margin-top:14px;background:#fff6ea}.practice-nav{display:flex;justify-content:space-between;align-items:center;max-width:850px;margin:20px auto;gap:10px}.privacy{text-align:center;font-size:14px}.text-button{padding:0;background:none;color:var(--blue);text-decoration:underline;min-height:35px;font-size:14px}footer{text-align:center;margin-top:45px;color:var(--muted);font-size:14px}[hidden]{display:none!important}@media(max-width:650px){header{padding:18px 18px}.brand span{font-size:13px}main{padding:15px}.intro{padding-top:10px}nav{gap:4px}nav button{padding:9px 10px;font-size:14px;flex:1}.section-top{align-items:start;flex-direction:column;gap:12px}.lesson-grid,.visual{grid-template-columns:1fr}.quiz{padding:21px}.quiz h3{font-size:22px}.worked{padding:0 18px 20px 25px}summary{padding:17px 15px}.select-label{align-items:start;flex-direction:column;gap:5px}.practice-nav{font-size:14px}.practice-nav button{padding:10px}.score{font-size:14px}h2{font-size:24px}}@media(prefers-reduced-motion:reduce){*{transition:none!important}}@media print{header,.intro,nav,.section-top button,.select-label,footer,#learn,#practice{display:none!important}main{max-width:none;padding:0}#answers{display:block!important}details{break-inside:avoid;border:0;border-bottom:1px solid #ddd;border-radius:0}summary:before{display:none}.worked{padding:0 12px 15px}body{background:white;font-size:11pt}.result{background:white}.page-title{break-after:avoid}}