How much of an apple pie has been eaten? The answer to this question can be expressed in percentages, 50%; or in decimals, 0.5; or in fraction, ½. In other words, half of mom's delicious apple pie is gone.
Review subtraction of 2 numbers whose sums would be 18 or less
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.
It is amazing the difference in effort you will get from worksheet to worksheet. Granted the amount of effort may vary immensely from year to year depending on the group of students you have.