[madnak]

Answers:

<font color="white">1 is 15. You've gone 2n miles. You used n/30 gallons to go the first n miles, and n/10 gallons to go the second n miles. So 4n/30 gallons to go 2n miles, 4n/60 to go n miles, 1/15 to go 1 mile, 1 gallon to go 15 miles. 15mpg.

2 is 60. Over a year, you'll go n miles. You'll use n/20 gallons to do so with the first car, and n/30 gallons with the second car. So with the second car, you're using 2/3 as much gas and saving $500. Thus, you're spending $1500 on gas with the first car, and $1000 with the second. To save an additional $500 from the second car, you need to halve the gas cost from $1000 to $500, and thus double the efficiency from 30mpg to 60mpg.</font>