An extra-strength aspirin contains 0.500 g of aspirin. how many grains is this? (1 grain = 64.8 mg)

The answer is 7.72 grains, this means between 7 and 8 grains.

Explanation:

First you must have the mass in the same units, that is in g or mg.   In this case the unit chosen is g. So they will convert the mass that contains a grain to g. For that you use the following Rule of Three:

If 1 mg  represents 0.001 g, then how many grams are 64.8 mg?

Now, to calculate how many grains represent 0.5 g, a new Rule of Three can be used:

If 1 grain  represents 0.0648 g, then how many grains are 0,5 g?

So the answer is 7.72 grains.

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