Algebra question (how they get the answer?))?

Question

A car is making a 12-mile per trip. It travels the first 8.0 miles at 30 miles per hour and the last 4.0 miles at 60 miles per hour. What is the car's average speed for the entire trip?

(Answer on the back of book: 36 mph)

but i don't know how they got it..

Answer 1

The average speed is (total distance) / (total time).

Total distance = 12 miles.

Total time = 8/30 + 4/60 hours = 1/3 hours.

Then d/t = 36 mph.

Answer 2

look, the formulae to solve this is a self derived one-

2ab/ a+b [here a and b are the tow speeds]

just feed the formula with the numbers-

2 (30*60)/ 30+60 [ / means division and * means multiply ]

=> 3600 / 90 = 40 !

the answer should be 40!

i hope i helped.. tc.

Answer 3

If the car was traveling at 30mph for 8 miles the total time for that is .26667 of an our by taking 8/30. The same for the change in speed when the car goes from 60mph at the 8 mile mark, and that total time is also in the form of a proportion which is 4/60 which is .0667 of an hour. Average speed can be seen as total distance/ total time that would be 12/(.2667+.0667) which equals 36.