Question

How many of the first 200 positive integers are multiples of neither 6 nor 15

Answer 1

There are
.. floor(200/6) = 33 multiples of 6
in the range 1 .. 200.

There are
.. floor(200/15) = 13 multiples of 15
in the range 1 .. 200, half of which are also multiples of 6.

So, there are 33 +13 -6 = 40 numbers in the range 1 .. 200 that are multiples of either 6 or 15 or both.

The remaining 200 -40 = 160 numbers are not multiples of either.