How many integers between 1 and 40 are multiples of 3 or 7?

I don't care for the answer, since it already told me. I just need a solution on how to answer these questions.

Question

How many integers between 1 and 40 are multiples of 3 or 7?

I don't care for the answer, since it already told me. I just need a solution on how to answer these questions.

anonymous · Answer

if it was me, i would count

anonymous · Answer

But it would take so long. Is there any method you can use to solve these problems?

anonymous · Answer

just make sure not to count 21 twice

anonymous · Answer

3 goes in to 40 13 times 
7 goes in to 40  5 times
but it is not 13+5 because you do not want to count 21 twice

anonymous · Answer

Oh, now I understand. I count 21 twice. My bad. 
Thank you very much!

anonymous · Answer

and i would disagree about it taking a long time, 
$$3,6,9,12,15,18,21,24,27,30,33,36,39,7,14,28,35$$ done

anonymous · Answer

How do I make sure that I don't count any number(s) twice?