Rafael Drive 405 miles using 20 gallons of gas. At that rate, how many gallons of gas would he need to drive 243 miles?

This is how I'd set it up:

405/20 = 243/x

this means the mpg (miles / gallons) for 1 situation is the same as the mpg for the other situation (because the rate is the same)

then just cross multiply and solve:

405x=243*20 or 405x=4860 or x = 12

he would need 12 gallons of gas for 243 miles