Sum Between Numbers

To find the sum of all numbers between two values (inclusive), add the first and last numbers, multiply by how many numbers there are, then divide by 2.

Sum = (first + last) × count ÷ 2

1 to 10

First + Last = 1 + 10 = 11

Count = 10 − 1 + 1 = 10

11 × 10 ÷ 2 = 55

5 to 15

First + Last = 5 + 15 = 20

Count = 15 − 5 + 1 = 11

20 × 11 ÷ 2 = 110

Don't forget to add one when calculating the count to include both start and end digits!