First, students learn what they are supposed to learn and when they are expected to do so. This is a word problem whose solution includes solving a system of two linear equations. Either way one looks, this is an undeniably an algebraic problem. Now, the problem can be solved by pure arithmetic. The applet below is supposed to help you out with this task.
All numbers are clickable.
They may be increased or decreased by clicking a little off their center lines. Check that this is indeed the way to manipulate the numbers, by setting the right sides to 17 and 3, respectively. The problem can be solved by counting. Take any two numbers, say 7 and 5. Note that increasing the first number increases both their sum and their difference.