3 devils and 3 humans are crossing a river in a boat.
Boat can carry only 2 things at a time.
As long as there are equal number of devils and humans, devils will not eat humans.
Now devils and humans are one side of the river, they have to go to other side.
Note: If one devil is at other side of the river and if a human accompanies a devil to that side, then devils will eat him up.
How to cross the river now?
First, two devils go across to the other side of the river, then the rower gets called back. Next, the rowing devil takes the second across and then gets called back, so now there are two devils on the far side.
Two humans go over, then one human accompanies one devil back, so now there is one human and one devil on the far side.
The last two humans go over to the far side, so now all the humans are across the other side, along with the boat and one devil.
In two trips, the devil on the far side takes the boat and ferries the other two devils across the river.
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