Rates
Rates play an important part of the Calculator Applications tests. These quantities are in the form of $\dfrac{\text{something}}{\text{something else}}$. Many times, the denominator of the rate is 1 and a unit of time. For example, 60 mph is 60 miles per 1 hour. Other examples include the cost of gasoline (\$2.47 per gallon), angular velocity (1.34 rad/s), and auger speed (87.2 bushels/min).
The key to working with rates is to make sure the units line up so that when they are in different parts of a fraction, they will cancel. Always take note of the units used in the final answer and work toward those units.
Example
A car has a speedometer in miles per hour, but is being driven in Canada, where they use kilometers per hour. If the speed limit is posted at 94 kph, what speed should the car go in miles per hour?
mph
Solution

We need to convert the rate given in kilometers per hour into miles per hour. Use the units conversions to make this change, canceling units as you multiply.

$\dfrac{94\ \text{km}}{\text{hr}} \times \dfrac{1000\ \text{m}}{1\ \text{km}} \times \dfrac{100\ \text{cm}}{1\ \text{m}} \times \dfrac{1\ \text{in}}{2.54\ \text{cm}} \times \dfrac{1\ \text{ft}}{12\ \text{in}} \times \dfrac{1\ \text{mi}}{5280\ \text{ft}} = 58.4\ \text{mph}$

Notice that conversion between miles and kilometers is one of the more common conversions needed on the Calculator Apps test. These conversion factors above can be approximated to 1.609. Using 1.609 km = 1 mi, the percent error is $-0.0214\%$.

Using 1.609 km = 1 mi, the calculation becomes $\dfrac{94\ \text{km}}{\text{hr}} \times \dfrac{1\ \text{mi}}{1.609\ \text{km}} = 58.4\ \text{mph}$.