The family and the boat
✓ With solution
A father, a mother, two children and a boat that carries no more than 80 kg. How do they all get across?
A family needs to cross a river in a small boat. The father weighs 80 kg, the mother 80 kg, and each of their two children 40 kg. The boat can carry no more than 80 kg at a time, and it only moves with someone in it rowing. They can all row.
What is the smallest number of crossings (each way counts) to get all four to the other side?
- A5
- B7
- C9
- D11
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Answer: C) 9. Nine crossings. The key is that the two children together weigh 80 kg, so they can cross together, but each parent has to cross alone. (1) Both children cross, (2) one comes back, (3) the father crosses, (4) the other child comes back, (5) both children cross, (6) one comes back, (7) the mother crosses, (8) the other child comes back, (9) both children cross. Each parent costs four crossings, to get them over and the boat back to the children, plus one last crossing for the children: 4 + 4 + 1 = 9.
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