As a middle-school math teacher, I have seen many students who do understand abstract concepts, and even enjoy learning about things like Pi and the Pythagorean Theorem, but who appear to be unable to do so due to a weak grasp of math fundamentals.
Review subtraction of 2 numbers whose sums would be 18 or less
Great, fun and free math worksheets should be able to present a mathematical problem in different ways. Math is after all nothing more than a numeric expression of some of life's simplest questions:
How much money do I have left if I buy a soda? By the end of the week, how much of my daily allowance will I be able to save if I don't?