Time, distance problem

Question

A family is on a road trip from Los Angeles to San Francisco. For the first 20 miles, they averaged 40mph, For the next 300 miles they averaged 75mph. If they drive 80mph for the next hour, what would their average speed be for the entire trip?

Answer 1

 

 

Distance = rate x time

 

Divides the trip into 3 parts as suggested in the problem.

 

1st 20 miles......

20 = 40 mph * t ---> t = 20/40 = 1/2 hour

 

next 300 miles.....

300 = 75 mph x t ---> 300/75 = 4 hours

 

last part.....

distance = 80 mph x 1 hour = 80 miles.

 

Total distance: 20 + 300 + 80 = 400 miles

 

Total time:  1/2 hour + 4 hours + 1 hour = 5.5 hours

 

Average speed = 400 miles/ 5.5 hours =  72 and 8/11 mph