Chemistry: The Central Science (13th Edition)

This question refers to converting units, and asks how we decide which part of the conversion factor is in the numerator and which is in the denominator. What you have to do is arrange the factor so that the current unit cancels, leaving you with the desired unit. For example, if you had $\frac{miles}{hour}$ and wanted to convert to $\frac{miles}{minute}$ you would use the conversion factor $\frac{1 hour}{60 minutes}$, with hours being on the top, as it would cancel hours in the original number, leaving you with the desired unit.