How many numbers between -13 and 13 are multiples of 3 or 5?

The correct answer is Option (2) → 13

To count numbers that are multiples of 3 or 5, count each kind, then subtract the numbers counted twice (those that are multiples of both, that is, of 15). Zero is a multiple of every number, since \(0=3\times0=5\times0\).

• Multiples of 3 between −13 and 13: 9 numbers; multiples of 5: 5 numbers

• Multiples of 15 in this range: only 0, which is in both lists

• Total \(=9+5-1=13\)

Tip: Zero belongs to both lists here, so it must be counted once; adding the two lists without removing it gives 14, which is Option (4).