On a drive from your ranch to Austin, you

wish to average 48 mph. The distance from

your ranch to Austin is 78 miles. However,

at 39 miles (half way), you find you have

averaged only 36 mph.

What average speed must you maintain in

the remaining distance in order to have an

overall average speed of 48 mph? Answer in

units of mph.

I thought the answer would be 60MPH but apparently that's wrong. Do I have to factor time into this equation?