How many natural numbers below 660 are divisible by 5 and 11 but not by 3?
8 of the numbers less than 660 are divisible by 5 and 11 but not
3.
All numbers divisible by 5 and 11 are multiples of their lcm;
lcm(5, 11) = 55
All numbers divisibile by 5, 11 and 3 are multiples of their
lcm; lcm(5, 11, 3) = 165
659 ÷ 55 = 11 54/55 → 11 numbers less than 660 are divisible by
5 and 11
659 ÷ 165 = 3 164/165 → 3 numbers less than 660 are divisible by
5, 11 and 3
→ of the 11 numbers less than 660 divisible by 5 and 11, 3 are
also divisible by 3
→ 11 - 3 = 8 numbers less than 660 are divisible by 5 and 11 but
not 3,.