She wonders if she will have to get gas before then. She normally sets her cruise control at the speed limit of 70 mph and the freeway portion of the drive takes about
an hour and 15 minutes. Her car gets about 30 miles per gallon on the freeway, and
gas costs $8.50 per gallon

Solution

To estimate the amount of gas she needs, Felicia calculates the distance traveled at 70 mph for 1.25 hours. She might calculate
70⋅1.25=70+0.25⋅70=70+17.5=87.5 miles.
Since 1 gallon of gas will take her 30 miles, 3 gallons of gas will take her 90 miles, a little more than she needs. So she might figure that 3 gallons is enough.

Or, since she is driving, she might not feel like distracting herself by calculating 0.25⋅70 mentally, so she might replace 70 with 80, figuring that that will give her a larger distance than she needs. She calculates
80⋅1.25=80+14⋅80=100.
So at 30 miles per gallon, 313 gallons will get her further than she needs to go, so should be enough to get her to the gas station.
Since Felicia pays $3.50 for one gallon of gas, and one gallon of gas takes her 30 miles, it costs her $3.50 to travel 30 miles. $3.5030 miles≈$0.121 mile, meaning it costs Felicia 12 cents to travel each mile on the freeway.