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