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?
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