/* =========================================================
   Block: § 01 / PROBLEM  (.problem)
   6 cards in a 3-col grid with collapsed borders.
   ========================================================= */
.problem {
  padding: 48px 0 120px;
  background: var(--ink-50);
  border-bottom: 1px solid var(--ink-100);
}

.problem-grid {
  display: grid;
  grid-template-columns: repeat(12, 1fr);
  gap: 0;
  border-top: 1px solid var(--ink-200);
  margin-top: 48px;
}
.problem-card {
  grid-column: span 4;
  padding: 40px 32px;
  border-right: 1px solid var(--ink-200);
  border-bottom: 1px solid var(--ink-200);
  position: relative;
  transition:
    transform .35s cubic-bezier(.2, .7, .2, 1),
    background .25s ease;
}
.problem-card:nth-child(3n) { border-right: none; }
.problem-card:hover {
  transform: translateY(-4px);
  background: rgba(255, 255, 255, .6);
}

.problem-index {
  font-family: var(--font-mono);
  font-size: 11px;
  color: var(--ink-400);
  letter-spacing: .1em;
  margin-bottom: 32px;
}
.problem-title {
  font-family: var(--font-serif);
  font-size: 24px;
  line-height: 1.25;
  font-weight: 500;
  letter-spacing: -.01em;
  color: var(--ink-900);
  margin: 0 0 14px;
}
.problem-body {
  font-size: 15px;
  line-height: 1.6;
  color: var(--ink-500);
  margin: 0;
}

/* ---------- Responsive ---------- */
@media (max-width: 1100px) {
  .problem-card { grid-column: span 6; }
  .problem-card:nth-child(3n) { border-right: 1px solid var(--ink-200); }
  .problem-card:nth-child(2n) { border-right: none; }
}

@media (max-width: 900px) {
  .problem-grid {
    grid-template-columns: 1fr;
    gap: 0;
    border-top: 1px solid var(--ink-100);
  }
  /* Reset every cascading nth-child border rule from tablet
     breakpoint — leave only a horizontal separator between cards. */
  .problem-card,
  .problem-card:nth-child(3n),
  .problem-card:nth-child(2n) {
    grid-column: auto;
    padding: 24px 20px;
    border: none;
    border-bottom: 1px solid var(--ink-100);
  }
  .problem-card:last-child { border-bottom: none; }
  .problem-title {
    font-size: 22px;
    line-height: 1.15;
    margin: 10px 0 12px;
  }
  .problem-body {
    font-size: 15px;
    line-height: 1.55;
  }
}