Find the next number in the pattern 1, 3, 6, 10, 15, __?

Step-by-step solution

1. List the differences: $3-1=2$, $6-3=3$, $10-6=4$, $15-10=5$.
2. The differences form an increasing sequence: $2,3,4,5,dots$
3. The next difference is $5+1=6$.
4. Add this to the last term: $15+6=21$.
5. Therefore the missing number is $21$.

Concept tested: Triangular number pattern

Published 1 October 2026 · How this solution was prepared and checked