The answer is 65. You just add the number of computer towers that fall into that category. So your sum would look like this:
Divisor: 5+ 12 + 21 + 15 + 12 = 65
The question does not ask you to do any more than figure out what you will be dividing by. Thank goodness it does not ask what you will be dividing into which is a whole lot more complicated problem.
