Use $5{,}280$ feet $=1$ mile. What can you multiply $\frac{15\text{ miles}}{1\text{ hour}}$ by so that the mile units cancel and you get feet on top?

After you have feet per hour, use $3{,}600$ seconds $=1$ hour to turn hours into seconds. Make sure the hour units cancel and check whether you should multiply or divide by $3{,}600$.

You should end up with a fraction like $\frac{79{,}200}{3{,}600}$. Try canceling zeros first, then reduce the smaller numbers step by step.

In a new expression line, type (15*5280)/3600. The value that Desmos displays is the car’s speed in feet per second.

The car’s speed is $15$ miles per hour, which we can write as the fraction

We want to change this to feet per second: $\frac{\text{feet}}{\text{second}}$.

Use $5{,}280$ feet $= 1$ mile to replace miles with feet.

Multiply by a conversion factor that equals 1:

The “miles” units cancel, leaving

So the speed is $79{,}200$ feet per hour.

Now convert hours to seconds using $3{,}600$ seconds $= 1$ hour.

To get feet per second, we need seconds in the denominator, so we divide by $3{,}600$ seconds per hour:

Now the unit is feet per second, as desired.

Simplify

First, cancel two zeros from top and bottom:

Now divide both numerator and denominator by 4:

Then divide $198$ by $9$:

So the car’s speed is $22$ feet per second, and the correct answer is $22$.