Convert $45$ miles per hour to feet per second.

Use the conversion factors: $1$ mile $= 5,280$ feet and $1$ hour $= 3,600$ seconds:

$45 \; \frac{\text{mi}}{\text{hr}} \times \frac{5,280 \; \text{ft}}{1 \; \text{mi}} \times \frac{1 \; \text{hr}}{3,600 \; \text{s}}$

$= \frac{45 \times 5,280}{3,600} = \frac{237,600}{3,600} = 66 \; \text{ft/s}$