Two rangers, two scouts, one small canoe
Two forest rangers, each weighing 85 kg, and two scouts, each weighing 40 kg, must all cross a river. Their only boat is a canoe with a strict load limit of 90 kg. Everyone can paddle, and the canoe obviously cannot cross by itself.
Notice what the limit allows: two scouts together weigh 80 kg (fine), one ranger weighs 85 kg (fine), but a ranger plus a scout is 125 kg (too heavy), and two rangers is out of the question.
Get all four across the river.
Your answer
This one is open-ended. Work it through, then check your reasoning against the full solution.