You plan to take a 1010-mile trip in your car, which averages 20 miles per gallon. How many gallons of gasoline should you expect to use? Would a car that has only half the gas mileage require twice as much gasoline for the same trip?
1010 / 20 =50.5 gallons of gas expected.
The second part of your question is: YES!. If the car does 10 miles to the gallon, then it is a simple proportion:
1010 / 10 =101 gallons required, which is TWICE the above answer.