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.
