Answer:
11.43 gallons used exactly per week asuming not changes in schedule or any other reasons to drive and that a one way trip is 24 miles.
Step-by-step explanation:
48 as total for two trips
five of these need to be done, so 240 miles a week assuming no other reason to drive
240 / 21 = 10 with remainder 30
so 11 and remainder 9
9/21 = nearly .43 gallons, which means as a minimum she shouldn't go below 11.5 and better yet 12 gallons.