A family of 17 are trying to get back to their island for Christmas when disaster strikes - their ship hits a rock and will sink in 20 minutes! They have a rescue boat that fits 5 people and can get to the island and back with it in 9 minutes. How many people are able to reach the island without getting wet before the ship sinks?

Answer: 13

Althoug each round trip takes 9 minutes and can carry 5 people on the boat, you can not just divide the 20 minutes time by 9 and multiply by 5 to find the number of people that are saved without getting wet.

First trip:

5 people take the boat to the island but only 4 can stay, because one has to return with the boat to the ship to rescue other 4 people.

So, when 9 minutes have passed, 4 people are safe on the island and the boat is again by the ship.

Second trip:

In the second trip, other 4 people are rescued, along with "sailor" who returned in the boat to rescue them.

When it has passed 18 minutes, the boat is next to the ship agin to carry other 4 people before the boat sinks.

This is the last trip before the boat sinks. So, the total number of passengers rescued is 4 + 4 + 5 = 13