
Answer:
 30 miles
Step-by-step explanation:
Let d represent the distance to the island. The relation between time, speed, and distance is ...
 time = distance/speed
so the total time for the round trip is ...
 d/15 +d/10 = 5
Multiplying by 30, we get
 2d +3d = 150
 d = 150/5 = 30
The island was 30 miles from the harbor.