How many positive integers less than 2020 aren’t divisible by 2, 3, 5, or 7?
We can use PIE and complementary counting. The numbers that ARE divisible by 2,3,5,7 are going to be (divisible by 2)+(divisible by 3)+(divisible by 5)+(divisible by 7)-(divisible by 2*3)-(divisible by 2*5)-(divisible by 2*7)-(divisible by 3*5)-(divisible by 3*7)-(divisible by 5*7)+(divisible by 2*3*5)+(divisible by 2*3*7)+(divisible by 2*5*7)+(divisible by 3*5*7)-(divisible by 2*3*5*7). This yields 1010+673+404+288-336-202-144-134-96-57+67+48+28+19-9=1559
:D