The C cars in a car service use a total of G gallons of gasoline per week. If each of the cars uses the same amount of gasoline, then, at this rate, which of the following represents the number of gallons used by 5 of the cars in 2 weeks?

A) 10cg

E)10g/c

Thanks to one of your methods, I first tried using c=1 and randomly chose 5 gallons to plug in. But then I got A, which was wrong. I later used c=2 and g=5, but then I got E, which is the right answer? Why doesn’t c=1 work? Thank you

Read my method more carefully—I’m very clear that you should never plug in 1, for exactly this reason. Look at the two choices you landed on! They’re exactly the same when *c* = 1.

When you plug in, remember that you must check every choice, and that if two choices work, you must plug in different numbers to get down to just one choice. You can avoid this happening to you *most* of the time if you don’t plug in 1 or 0.

Try using 20 for *c* and see what happens.