Find the sum of all natural numbers divisible by 7 and not exceeding 400.

1. Determine the maximum number multiple of 7 and less than 400.

400/7 = 57.14. So the maximum number we need is 7 * 57 = 399;

2. Find out the sum of the first 56 numbers from 7 to 392;

If we add the first number in row 7 and the last 392 we get 7 + 392 = 399;

The second number 14 and the penultimate 385 will also add up to 14 + 385 = 399;

There will be 56/2 = 28 such pairs. So the sum of numbers from 7 to 392 is 399 * 28 = 11172. Let’s add the last number and get 11172 + 399 = 11571;

Answer: 11571;