81 → 729 → 9 36 → 216 → 6 16 → ? → 4 We have to find the missing number (?).

In each triad here, the three numbers are built from one base number n in a fixed order: the first term equals n², the second term equals n³, and the third term is n itself — that is, the pattern reads n² → n³ → n.

1. First triad: 81 → 729 → 9. Here the base is 9, since 9² = 81 and 9³ = 729.

2. Second triad: 36 → 216 → 6. Here the base is 6, since 6² = 36 and 6³ = 216.

3. Apply the same rule to the third triad: 16 → ? → 4. Here the base is 4, since 16 = 4².

4. So the missing term must be 4³ = 4 × 4 × 4 = 64.

Cross-check: 4³ = 64 by direct multiplication, and the full triad 16 → 64 → 4 matches the same n² → n³ → n structure confirmed in both earlier triads.