Answer:
t = 0.53 hr
Explanation:
We write expressions for the distances walked by these two people, sum them up and set the sum equal to 4 miles:
(3.5 mph)(t) + (4mph)(t) = 4 mi
Combining these terms:
(7.5 mph)(t) = 4 mi
4 mi
Solving for t: t = ---------------- = 0.53 hr
7.5 mph