Why are we not taking 15 and 4? Since its sum is 19 and we already have 3+16 =19. But where is it written that sum of multiples of 3 and 4 must be distinct?

The question is how many positive integers are the sum of …

So it is asking for numbers that are a sum of 3x and 4y

Not the x and y

Understood but why can’t we repeat 19?

All numbers from 1 to 20: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20

How many of them are divisible by sum of a multiple of 3 and a multiple of 4?

1 Like