If you would like to know which unit rate is equivalent to 17 miles per gallon, you can calculate this using the following steps:
17 miles ... x gallon = ?
51 miles ... 3 gallons
x = 17 * 3 / 51 = 1 gallon
17 miles ... x gallon = ?
34 miles ... 4 gallons
x = 17 * 4 / 34 = 2 gallons
The correct result would be: fifty-one miles over three gallons.