Сколько существует целых положительных чисел, меньших 100, которые делятся на 3, но не делятся на 5?

The answer is 27. 

Solution: 

A - number of numbers, that divides by 3

B - number of numbers, that divides by 15, or in other words - number of numbers, that divides by 5 and 3 at the same time.

Z - number of numbers, that divides by 3 and dont divides by 5.

[] - in my solution this means lower complete value, e.g [5/3] = 1.

 

 

A = [100/3] = 33

B = [100/15] = 6

 

Z = A - B