Answer
The car must travel at 64 miles per hour to completely pass the truck in 5 seconds.
Work Step by Step
The total distance the Ford must cover to completely pass the truck is the truck's length plus its own length. That means 50+16=66 feet.
The formula for speed is $v=\frac{d}{t}$ So, if it wants to pass the truck in 5 seconds, it must be $v=\frac{66}{5}$ feet per second faster than the truck. That means:
$v_t=\dfrac{55 \text{ miles}}{\text{ hour}}+\dfrac{66 \text{ feet}}{5 \text{ seconds}}$
The units aren't the same, so we must convert them. In this case, we'll want the final units to be in miles per hour. There are 3600 seconds in an hour and 5280 feet in a mile, so:
$v_t=\dfrac{55 \text{ miles}}{\text{ hour}}+\dfrac{66 \text{ feet}}{5 \text{ seconds}}\cdot \dfrac{3600 \text{ seconds}}{1 \text{ hour}}\cdot \dfrac{1 \text{ mile}}{5280 \text{ feet}}$
$v_t=\dfrac{55 \text{ miles}}{\text{ hour}}+\dfrac{237600 \text{ miles}}{26400 \text{ hours}}$
$v_t=\dfrac{55 \text{ miles}}{\text{ hour}}+\dfrac{9 \text{ miles}}{ \text{ hour}}=64$ miles per hour
