First of all, lets ask ourselves how we are going to get Mary's marbles. In the problem, we can see that Joan has 15 times as many marbles as mary. So this means that Mary's marbles are multiplied to 15 in order to get Joan's marbles (which happens to be 45). So if we reverse the situation, then in order to get Mary's marbles, you are going to have to divide Joan's marbles by 15. Since we know that Joan has 45 marbles, we divide that by 15. The answer is 3. Mary has a total of 3 marbles.
