What is the sum of all numbers from 21 to 30, inclusive?

Step-by-step solution

1. Identify the numbers as an arithmetic series with first term $a=21$ and last term $l=30$.
2. Count the terms: $30-21+1=10$ terms.
3. Use the sum formula $S=\frac{n}{2}(a+l)$.
4. Compute $S=\frac{10}{2}(21+30)=5\times 51=255$.
5. Result is $255$.

Concept tested: Arithmetic series sum

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