shankar245 wrote:

Karishma,

Why is this not working

?

12*(2/3) + 18 *(1/3)

/1

14??

We need to find the average miles per gallon.

Average miles/gallon = Total miles/Total gallons

What is the distance travelled (i.e. miles)?

"traveled twice as many miles from Town A to Town B as it did from Town B to Town C"

We know that the ratio of the two distances must be 2:1 or we can say the distances must be 2d and d.

What is the total gallons used?

Fuel used to go from A to B = 2d/12

Fuel used to go from B to C = d/18

So Average miles/gallon = (2d + d)/(2d/12 + d/18)

Go back to my post above. It explains that you have to be mindful of how to use weighted averages. You need to know what the weights are going to be. Here, weights have to be the amount of fuel used i.e. in gallons because you are looking for average miles per gallon.

If we change the question and make it:

A certain car used twice as many gallons from Town A to Town B as it did from Town B to Town C. From Town A to Town B, the car averaged 12 miles per gallon, and from Town B to Town C, the car averaged 18 miles per gallon. What is the average miles per gallon that the car achieved on its trip from Town A through Town B to Town C?

Now you can use 12*(2/3) + 18 *(1/3)/1

I hope it makes sense now.

_________________

Karishma

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