A student wants to know how many integers between 1 and 1000 are a multiple of 4 or a multiple of 5. She wonders if it is correct to find the number of those integers that are multiples of 4 and add the number of those that are multiples of 5. How do you respond?
That is not a correct method, because some numbers are multiples of both 4 and 5. Those are the ones divisible by 20.
The correct answer is 250 + 200 - 50 = 400.
Do you see why?