Answer:
3/4 hour
Step-by-step explanation:
The ratio of speeds going : coming is 9 : 3 = 3 : 1. The ratio of times will be the inverse of this: 1 : 3.
The time spent walking back is 3/(1+3) = 3/4 of the total, or 3/4 hour.
Andrea should plan on walking back for 3/4 hour.