What is the sum of the first four multiples of five?

Correct answer: 50. The first four multiples of 5 are 5, 10, 15, 20, and 5 + 10 + 15 + 20 = 50.

Background: The first four multiples of 5 are 5, 10, 15, and 20. Their sum is 50.

• 25: 25 is too small.
• 55: 55 is not the sum of those numbers.
• 60: 60 is not the sum of those numbers.

Did you know? A shortcut formula: the sum of the first n multiples of k is k × n(n + 1) ÷ 2, so 5 × 4 × 5 ÷ 2 = 50.

Memory trick: 5 × (1 + 2 + 3 + 4) = 5 × 10 = 50.