I'll let others chime in with their thoughts, but question B is tricky - because both 201 and 501 are multiples of 3, you'd need to indicate whether the set is "inclusive" (the end values, 201 and 501, count) or "exclusive" (they don't).

For a question like A, you'd want to find the first and last multiple of 3 in the set and make sure to include both.

A quick way to remember the rules for inclusive/exclusive sets:

Just use 1, 2, and 3. The range of that set is 2, but you know that there are 3 values if you include them all, and only 1 value in between. So that proves that:

Inclusive set - take the range and add 1
Exclusive set - take the range and subtract 1

If your memory blanks, or you just don't want to memorize in the first place, you can just prove the rule (as long as you know that such a rule exists) with small numbers.