# John drove for two hours at the speed of 50 miles per hour (mph) and another x hours at the speed of 55 mph. If the average speed of the entire journey is 53 mph, which of the following could be used to find x?

##### 1 Answer

#### Answer:

#### Explanation:

The idea here is that you need to work backwards from the definition of the *average speed* to determine how much time did John spend driving at **55 mph**.

The average speed can be thought of as being the ratio between the *total distance* travelled and the *total time* needed to travel it.

At the same time, distance can be expressed as the product between velocity (in this case, speed) and time.

So, if John drove for **2 hours** at **50 mph**, then he covered a distance of

The second part of the total distance was travelled at **55 mph** for **x hours**, so you can say that

The total distance travelled is equal to

The total *time* needed was

This means that the average speed is

Solve this equation for