Complete the Number Series: 1, 3, 6, 10, 15, ?
Last updated Jun 13, 2026
Correct Answer:
Option C —
21
To find the missing number, we can look at how much the series increases with each step.The difference between one and three is two.
The difference between three and six is three.
The difference between six and ten is four.
The difference between ten and fifteen is five.
The amount being added increases by one each time. Following this pattern, the next difference needs to be six.
Adding six to fifteen gives twenty one.
These numbers are also known as triangular numbers, as they can be arranged in a meaningful geometric pattern of dots.
Answer verified by Quintessence Classes faculty — Karan Nagar, Srinagar.
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