How to solve this? The correct answer is A
Imagine the situation as two boats starting from the same point. One goes twice as fast as the other.
So, both start from Sandpoint at time 0:00. Hovercraft comes to Genville at 0:50 and stays there for 20 minutes, until 1:10 and then heads back.
Slow ferry isnt even in Genville yet, because it has only travelled 1:10 hours and has 30mins remaining to Genville. Thus, because hovercraft travels twice as fast as slow ferry, it will cover the same distance in 10mins and slow ferry in 20mins. In other words, hovercraft sails 2x distance while slow ferry only x.
The answer is when they meet, slow ferry has been travelling for = 1:10 + 0:10 hours = 1:20 hours.