On the first trip, 5 people are in the raft... but only 4 are left on the island, as 1 man
has to take the raft back to pick up the second set of people
On second trip, again 5 in the boat, only four left on the island with 1 man to go back to get the last few
Now you are 18 minutes --- only 2 more minutes til the boat sinks BUT you can get 5 people on the boat..... BEFORE it actually sinks...
so 4 + 4 + 5 = 13 in the 20 minutes...
Lets have a moment of silence for the 2 that didnt make it!