Question

Sam works two jobs, one at the zoo and the other at
the library. He drives 24 miles round-trip to the zoo
and 9 miles round-trip to the library. He never works
two jobs in the same day. His car has a 15 gallon
gas tank and gets 20 miles per gallon. If his tank is
now empty and he has worked 12 days at the library
since his last filling up, how many days did he work
at the zoo?

Answer 1

assuming the tank was originally full:

total amount of gas = miles per gallon * number of gallons = (15)(20)

gas spent on library days, total: number of days worked at the library * gallons used on a library day = (12 * 9)

furthermore:

gas spent at the zoo = total gas - library gas

then divide the result by the number of gallons spent per zoo day to get the total # of zoo days