Gabriella drives her car 300 miles and averages a certain speed. If the average speed had been 9mph less, she could have traveled only 240 miles in the same length of time. What is her average speed?

Her average speed is 45 mph

Step-by-step explanation:

Let the average speed be x mph

Time taken complete the journey = distance / average speed

Time = 300/x

Now, at 9 mph less, she could have only traveled 240 miles in same time

At 9 mph less, the average speed is x-9

so the time would be 240/x-9

since the time is same as before;

300/x = 240/x-9

Divide both sides by 60

5/x = 4/x-9

Cross multiply

5 (x-9) = 4 (x)

5x - 45 = 4x

5x - 4x = 45

x = 45 mph