Find the INCORRECT number in the series. 3, 6, 9, 13, 15, 18

Correct — C.
Rule: multiples of 3, rising by 3 each step.

3, 6, 9, then 9 + 3 = 12, but the series prints 13.
After it, 15 and 18 carry on as if 12 were there.

The gaps show it too: +3, +3, +4, +2, +3. One wrong term bends the two gaps on either side of it → option (c).

Why the others are wrong

• (a)
  6 — 6 is 3 × 2, the second multiple of 3, and both its gaps (3 → 6 and 6 → 9) are +3. Nothing about it breaks the rule.
• (b)
  9 — 9 is 3 × 3, the third multiple of 3, in its right place. The step 6 → 9 is exactly +3.
• (d)
  15 — 15 is 3 × 5, the fifth multiple of 3, and 15 → 18 is a clean +3. The gap 13 → 15 looks wrong only because 13 is.

In a wrong-term series, find the rule from the terms that agree, then test each term against it.

Here 3, 6, 9, 15 and 18 are all multiples of 3. That fixes the rule at +3, and the term that fails it, 13 in the place of 12, is the misprint.

A single wrong term shows up as two odd gaps, one before it and one after. The term the two odd gaps share is the one to mark.

Key facts

• The first six multiples of 3 are 3, 6, 9, 12, 15, 18.
• 13 stands in the fourth place, where 12 belongs.
• A single wrong term disturbs two gaps: here 9 → 13 is +4 and 13 → 15 is +2.

Common traps

• Marking 15 because the gap 13 → 15 is +2, when that gap is odd only because of 13
• Seeing two odd gaps and suspecting two wrong terms, when one term is enough to cause both

No directly related past PYQ was found.