Complete the Number Series: 1, 3, 6, 10, 15, ?

Reasoning Ability ·Previously asked in JKSSB Laboratory Attendant 2026

View the full solved paper: JKSSB Laboratory Attendant – 10 May 2026

Question

Complete the Number Series: 1, 3, 6, 10, 15, ?

  1. A. 18
  2. B. 20
  3. C. 21 (Correct answer)
  4. D. 22

Correct Answer

Option C — 21

Detailed Solution & Explanation

The correct answer is 21.

Key Points

  • These are the triangular numbers — the running sums 1, 1+2, 1+2+3, and so on.
  • Series: 1, 3, 6, 10, 15, ?
  • The differences increase by 1 each time: +2, +3, +4, +5, so the next difference is +6:
  • 15 + 6 = 21.
  • the nth triangular number is given by the formula n(n + 1)/2.
  • Here the 6th triangular number is 6 × 7 / 2 = 21, which matches.
  • Triangular numbers appear constantly in JKSSB and SSC series questions.

Exam Tip

  • memorise the first triangular numbers — 1, 3, 6, 10, 15, 21, 28, 36, 45, 55 — so you recognise the pattern on sight instead of computing differences each time.

Topics covered: Number Series