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guddo
A certain truck averages 10 miles per gallon when driven in the city and 25 miles per gallon when driven on the highway. According to these rates, which of the following is closest to the number of miles per gallon that the truck averages when it is driven 20 miles in the city and 30 miles on the highway?

A. 16
B. 18
C. 20
D. 23
E. 25

Attachment:
2024-01-03_18-44-47.png
Fuel used in city is 20/10 = 2
Fuel used in highway is 30/25 = 1.2

So, mileage is (30+20)/(2+1.2) = 15.xx ~ 16, Answer must be (A)
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Could someone explain to me why you can't use weighted averages to find the answer here?

Theoretically, shouldn't the answer be 2/5(10m/g)+3/5(20m/g)? Why doesn't this work?
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camryn111
Could someone explain to me why you can't use weighted averages to find the answer here?

Theoretically, shouldn't the answer be 2/5(10m/g)+3/5(20m/g)? Why doesn't this work?
Kindy refer to this explanation for a similar question on why weighted averages can't be used - https://gmatclub.com/forum/a-certain-car-averages-25-miles-per-gallon-of-gasoline-when-driven-in-128139.html#p1079109­­
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