Bunuel
An experimental vehicle averages \(x\) miles per gallon when driven in the city and \(2x\) miles per gallon when driven on the highway. If the vehicle averages 28 miles per gallon when driven 120 miles in the city and 300 miles on the highway, what is the value of \(x\)?
A. 14
B. 16
C. 18
D. 20
E. 28
Total gallons of fuel consumed:
\(\frac{120}{x} + \frac{300}{2x} = \frac{420}{28}\)
Bring to lowest terms:
\(\frac{120}{x} + \frac{150}{x} = 15\)
\(\frac{270}{x} = 15\)
\(x = 18\)
Answer (C)Though I wouldn't solve like this in the exam. I would use logic and intuition but for that, you need to understand weighted averages very well. I am putting down the intuitive method below.
Average miles per gallon is 28 so it lies between x and 2x. Now weights here are gallons, not miles so I cannot use weighted averages directly.
Just like with speed, we cannot use distance as weight; we must use time. If certain speeds are maintained for equal time, average is right in the centre. If certain speeds are maintained for equal distances, average is closer to lower value. So average worsens.
If the ratio of gallons were 120:300 i.e. 1:2.5, then the distance of 28 from x and 2x would be in the ratio 2.5 : 1. But ratios of miles is given to be 1:2.5. So ratio of distances of 28 from x and 2x will be less than 2.5 : 1.
x cannot be 14 because then 2x is 28 but average must be between 14 and 28.
If x is 16, 2x is 32 so average 28 would be in the ratio 3:1 away from x and 2x. This is higher than 2.5:1
If x were 18, 2x is 36 and average 28 is in the ratio 5:4 i.e 1.25:1 away from x and 2x. Possible.
If x were 20, 2x would be 40 and average is closer to 20. That's too much.
Hence answer is 18.