Please help Gmat prep 2 PS: a certain car averages..

[yichunette]

A certain car averages 25 miles per gallon of gasolin when driven in the city and 40 miles per gallon when driven on the highway. According to these rate, which of the following is closest to the number of miles per gallon that the car averages when it is driven 10 miles in the city and then 50 miles on the highway?

Answer: 36

Please explain how to solve this problem.

Thank you!

Yi

[rfernandez]

The first step is to figure out how much gasoline the car uses on each leg of the trip. We can find this by dividing the number of miles traveled by the miles per gallon.

City: 10 miles / 25 miles per gallon = 2/5 gallon
Highway: 50 miles / 40 miles per gallon = 5/4 gallon

So the car uses 2/5 + 5/4 gallons total, or 33/20 gallons.

The average miles per gallon for the entire trip can be calculated by dividing total miles by total gallons:

60 miles / (33/20) gallons = 400/11 mpg, or roughly 36 mpg.


Rey

[yichunette]

Thank you very much for your help!

[rfernandez]

You're welcome!

[floubin]

Why can't you solve this problem this way:

10/60 of the trip in the city, 50/60 of the trip on the highway

(1/6)*25 + (5/6)*40 = 37.5 or ~38

That would make the answer E, not D

?

[StaceyKoprince]

For these kinds of problems, you can't just calculate an average using the two rates given. The overall mpg is going to be (total mileage) / (total gallons), so you actually have to calculate total mileage and total gallons.