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.
Ratios and proportions are likewise wonderful math lessons with plenty of interesting practical applications. If three pans of pizza, one kilo of spaghetti, two buckets of chicken can properly feed 20 hungry friends, then how much pizza, spaghetti and chicken does mom need to prepare for birthday party with 30 kids?
However for the most part, when a worksheet is needed to help drill down a procedure, standard, or lesson, its effectiveness can and will vary.
There are several standard exercises which train students to convert percentages, decimals and fractions. Converting percentage to decimals for example is actually as simple as moving the decimal point two places to the left and losing the percent sign "%." Thus 89% is equal to 0.89. Expressed in fraction, that would be 89/100. When you drill kids to do this often enough, they learn to do conversion almost instinctively.
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