average speed for remaining distance, for overall ave of 64

[Math wiz ya rite 09]

On a drive from your ranch to Austin, you wish to average 64 mph. The distance from your ranch to Austin is 80 miles. However, at 40 miles (half way), you find you have averaged only 48 mph.

What average speed must you maintain in the remaining distance in order to have an overall average speed of 64 mph?

 

[tkhunny]

Distance = Rate * Time

Extended a bit

Total Distance = Average Rate * Total Time

Assemble the known pieces

Whole Trip: 80 mi = 64 mph * Total Time
First Half: 40 mi = 48 mph * First Time
Second Half: 40 mi = X mph * Second Time

Solve "First" and "Second" for time and substitute in the "Whole". You're almost done.