Answer:
7.75 hours
Explanation:
Distance = Rate times Time
Monica 400=50xT
Mary 300=40xT
Monica 400/50=T, so T=8 hours. This is how long it takes her to get home.
Mary 300/40=T, so T=7.5 hours. This is how long it takes her to get home.
The difference in time between 7.5 and 8 hours is 7.75 hours. This is the time they will be an equal distance from home.