What comes next: 3, 6, 18, 108, ?

Correct — B.
Rule: each term is the product of the two terms before it.

3 × 6 = 18
6 × 18 = 108
Next: 18 × 108 = 1944 → option (b).

Check by ratios: 6 ÷ 3 = 2, 18 ÷ 6 = 3, 108 ÷ 18 = 6, and 1944 ÷ 108 = 18.

Why the others are wrong

• (a)
  1290 — 1290 is not a whole multiple of 108 (108 × 11 = 1188, 108 × 12 = 1296), so no whole-number multiplier from 108 reaches it.
• (c)
  648 — 648 is 108 × 6, repeating the last multiplier. The multipliers grow, 2, 3, 6, and the next one is 18.
• (d)
  432 — 432 is 108 × 4, but the multipliers run 2, 3, 6 and never shrink. Multiplying the last two terms, 18 × 108, gives 1944.

When a series grows very fast, test multiplication before addition. Try two tests: a multiplier from term to term, and each term as the product of the two before it.

Here the multipliers 2, 3, 6 show no fixed step, but 3 × 6 = 18 and 6 × 18 = 108 fit the product rule exactly.

Key facts

• 3 × 6 = 18 and 6 × 18 = 108.
• The next term is 18 × 108 = 1944.
• The multipliers run 2, 3, 6, 18.

Common traps

• Repeating the last multiplier, × 6, and picking 648
• Testing only the differences (3, 12, 90) and finding no pattern

No directly related past PYQ was found.