Skip to content

Lesson eLearning

What is the sum of all numbers from 10 to 20 inclusive?

hard 5 marks Class 1 · Mathematics · Numbers from Ten to Twenty

Answer

165

The correct answer is 165 — Hence the sum is 165.

Step-by-step solution

Count the numbers: $20 - 10 + 1 = 11$ terms.
Use the arithmetic series sum formula: $S = \frac{( a + l ) n}{2}$ .
Here $a = 10$ , $l = 20$ , $n = 11$ .
Compute $S = \frac{( 10 + 20 ) \times 11}{2} = \frac{30 \times 11}{2} = 165$ .
Hence the sum is 165.

Common mistake: Forgetting to include both endpoints or miscounting the number of terms

Concept tested: Sum of consecutive integers

Published 7 October 2026 · How this solution was prepared and checked

← All Numbers from Ten to Twenty questions