汽油依赖

http://www.glassdoor.com/Interview/business-sense-There-are-two-types-of-cars-A-and-B-The-number-of-people-in-US-who-use-A-and-B-are-the-same-They-driv-QTN_831574.htm

 

business sense: There are two types of cars A and B. The

   number of people in US who use A and B are the same. They drive the same distances each month. Now there are two new technologies, X and Y (of equal cost). If apply X, mpg of A would increase from 50 mpg to 75 mpg; If apply Y, mpg of B would increase from 10 mpg to 11 mpg. The goal is to decrease the dependence on foreign oil, or to decrease the consumption of gasoline. Question: which technology would you apply? Follow up question: after applying the technology of your choice, assume there's money available for research on new technology, which car would you choose to conduct research on? 

Given:
A.people = B.people //Same num people
A.miles = B.miles //Same miles

A.gallons * A.mpg = B.gallons * B.mpg
A.gallons = B.gallons * B.mpg / A.mpg

total.gallons = A.gallons + B.gallons = B.gallons * B.mpg / A.mpg + B.gallons = B.gallons * (1+ B.mpg / A.mpg) = B.gallons * (1+10/50) = 1.2 * B.gallons
total.gallons = A.gallons * (1 + 50/10) = 6 * A.gallons

X.diff = ((1 + 10/50) * B.gallons - (1 + 10/75) * B.gallons) / ((1 + 10/50) * B.gallons) = 5.5% decrease
Y.diff = ((1+ 50/10) * A.gallons - (1+50/11) * A.gallons) / ((1 + 50/10) * A.gallons) = 7.5% decrease //Y has bigger impact