A family are out on a hike and reach a river which they need to cross. There is a canoe which can carry 100 kg.
The father weighs 90 kg, the mother weighs 80 kg, the elder child weighs 60 kg, the younger child weighs 40 kg, and they have a dog that weighs 20 kg.
Assuming that the humans can all paddle the canoe, how do they cross the river?
It is obvious that the father must travel alone, that the mother can take the dog, and the children can travel together. So the puzzle revolves around who takes the canoe back to minimise crossings.
Here is one variation:
The two children paddle across and the elder one returns
The father paddles across and the younger child returns
The two children cross again and the elder one returns
The mother takes the dog across and the younger child returns
The two children paddle over for the last time