This puzzle, known as the “IQ150 Puzzle,” is one I learned during middle school. It’s a fun yet challenging exercise that tests logical thinking and creativity. Despite its somewhat extreme setting, the puzzle is captivating and rewarding to solve.

Puzzle Setup

A family is trying to cross a large river. Here’s the structure of the family and the conditions of the puzzle:

Family Members

• Father👨
• Mother👩
• Two daughters👧👧
• Two sons👦👦
• Servant👩‍🦱
• Dog🐶

Conditions

1. There is only one boat, and it can hold a maximum of two passengers at a time (including the dog as one passenger).
2. The boat must have a rower to operate. Only the father, mother, and servant can row the boat.
3. Certain combinations of family members pose dangers:
   • The father will harm the daughters if the mother is not present.
   • The mother will harm the sons if the father is not present.
   • The dog will harm the family if the servant is not present.
4. The boat can make as many trips as necessary.

Can you devise a way to get everyone across the river safely?

Background of the Puzzle

This puzzle appears to be a variation of the classic “river crossing problem,” a logic puzzle thought to have been created by the 8th-century English theologian Alcuin of York.

A famous example of Alcuin’s puzzle is:

The Problem: A farmer needs to transport a wolf, a goat, and a cabbage across a river.
However, the following conditions must be met:

• The boat can hold only the farmer and one item at a time.
• If left alone, the wolf will eat the goat, and the goat will eat the cabbage.

The “IQ150 Puzzle” builds on this classic concept with a more complex and modern twist.

Example Solution

Let’s analyze the steps of the given solution to ensure everyone crosses the river safely.

1. The servant and the dog cross the river.
   → On the far side: Dog.
   → On the starting side: Father, mother, two daughters, two sons, servant.
   ✅ No violations of the conditions.
2. The servant returns.
   → On the far side: Dog.
   → On the starting side: Father, mother, two daughters, two sons, servant.
   ✅ No violations of the conditions.
3. The servant and the elder son cross.
   → On the far side: Elder son, dog.
   → On the starting side: Father, mother, younger son, two daughters, servant.
   ✅ No violations of the conditions.
4. The servant and the dog return.
   → On the far side: Elder son.
   → On the starting side: Father, mother, younger son, two daughters, servant, dog.
   ✅ No violations of the conditions.
5. The father and the younger son cross.
   → On the far side: Elder son, younger son.
   → On the starting side: Father, mother, two daughters, servant, dog.
   ✅ No violations of the conditions.
6. The father returns.
   → On the far side: Elder son, younger son.
   → On the starting side: Father, mother, two daughters, servant, dog.
   ✅ No violations of the conditions.
7. The father and mother cross.
   → On the far side: Father, mother, elder son, younger son.
   → On the starting side: Two daughters, servant, dog.
   ✅ No violations of the conditions.
8. The mother returns.
   → On the far side: Father, elder son, younger son.
   → On the starting side: Mother, two daughters, servant, dog.
   ✅ No violations of the conditions.
9. The servant and the dog cross.
   → On the far side: Father, elder son, younger son, servant, dog.
   → On the starting side: Mother, two daughters.
   ✅ No violations of the conditions.
10. The father returns.
    → On the far side: Elder son, younger son, servant, dog.
    → On the starting side: Father, mother, two daughters.
    ✅ No violations of the conditions.
11. The father and mother cross.
    → On the far side: Father, mother, elder son, younger son, servant, dog.
    → On the starting side: Two daughters.
    ✅ No violations of the conditions.
12. The mother returns.
    → On the far side: Father, elder son, younger son, servant, dog.
    → On the starting side: Mother, two daughters.
    ✅ No violations of the conditions.
13. The mother and the elder daughter cross.
    → On the far side: Father, mother, elder son, younger son, elder daughter, servant, dog.
    → On the starting side: Younger daughter.
    ✅ No violations of the conditions.
14. The servant and the dog return.
    → On the far side: Father, mother, elder son, younger son, elder daughter.
    → On the starting side: Servant, dog, younger daughter.
    ✅ No violations of the conditions.
15. The servant and the younger daughter cross.
    → On the far side: Father, mother, elder son, younger son, elder daughter, servant, younger daughter.
    → On the starting side: Dog.
    ✅ No violations of the conditions.
16. The servant returns.
    → On the far side: Father, mother, elder son, younger son, elder daughter, younger daughter.
    → On the starting side: Servant, dog.
    ✅ No violations of the conditions.
17. The servant and the dog cross.
    → Everyone successfully crosses to the far side.
    ✅ All conditions met.

Conclusion

This solution satisfies all the given conditions, ensuring that no one is harmed and everyone, including the dog, safely reaches the other side of the river.

This challenging puzzle exemplifies the importance of logical thinking and careful planning. For those seeking more mental exercises, exploring variations of the classic “river crossing problem” can provide hours of intellectual fun and stimulation!