Answer:3.5$
Step-by-step explanation:r. First we have to let the car travel the same distance before and after the modification.Suppose 210 miles
Before:150/5=30mile/gallon so we need 210/30=7gallons
After:210/6=35mile/gallon so we need 210/35=6gallons
The difference is 1 gallon.1 multipy by 3.5$/gallon=3.5$
So 3.5$ is Mr.saved.
( I am not sure if teh answer is correct.)