Use Your Knowledge of English: 

1. Analyze the Units of the Answer. How many dimensions?

Find the mass in grains of a 325 milligram aspirin tablet.
(Given: 1.00 g = 15.4 grains)

grains are a mass unit: a single (simple) dimension!

2. Is there a single dimension 'number and unit' not connected to a Unit Factor in the problem?

"325 milligram aspirin tablet" is also a single dimension
of mass which can be connected to the Single Dimension Mass Unit of the Answer!

3. Equate the given to the unit asked for in the answer.

_???__grains = 325 milligram tablet