In the sequence A, C, F, J..., which letter is the 5th?

Correct — A.
Rule: the gaps grow by one: +2, +3, +4, +5.

A(1) + 2 = C(3)
C(3) + 3 = F(6)
F(6) + 4 = J(10)

The next gap is +5: J(10) + 5 = O(15) → option (a).

Why the others are wrong

• (b)
  P — P is position 16, six places after J. A gap of +5 skips only four letters (K, L, M, N), so skipping five letters after J (K to O) overshoots to P.
• (c)
  N — N is position 14, J + 4. That repeats the last gap instead of growing it; the gaps run +2, +3, +4, so the next one is +5.
• (d)
  M — M is position 13, J + 3, which reuses an earlier gap. The gaps must keep growing, so M falls two letters short of O.

A single-letter series is a number series in disguise. Turn each letter into its place in the alphabet (A = 1 … Z = 26) and look at the gaps between terms.

Here the positions are 1, 3, 6, 10. The gaps are 2, 3, 4, each one more than the last. Continue that and the fifth term sits 5 places after J, at O.

Key facts

• E = 5, J = 10, O = 15, T = 20 and Y = 25 are quick anchors for letter positions.
• The gaps here are +2, +3, +4, +5, giving A, C, F, J, O.
• The positions 1, 3, 6, 10, 15 are triangular numbers: 1 + 2 + … + n.

Common traps

• Skipping five letters after J and landing on P: a +5 jump skips only four
• Repeating the last gap (+4) instead of growing it, which gives N

No directly related past PYQ was found.