If I drive a car, and the first minute average $40$ mpg, average $30$ mpg the second minute, and average $36$ mpg the third minute, what mpg must I average the fourth minute to get an overall average of $38$ mpg?
This question is very tricky I think because we aren't given the speed or any other information about the car. Any solution would help.
You don't have enough information to solve the problem. During the second minute you might have stopped the car, driven essentially no distance, and used essentially no gas.
You might be expected to assume that you are driving at a constant speed, so at the same distance per minute. In that case, let $d$ be the distance per minute and $m$ the mileage in the fourth minute. Your total consumption is $\frac d{40}+\frac d{30}+\frac d{36}+\frac dm=\frac{4d}{38}$ The $d$'s divide out and you are left with an equation in $m$