A woman is traveling with a wolf, a goose, and a mouse. She must cross a river in a boat that will only hold herself and one other animal. If left to themselves, the wolf will eat the goose and the goose will eat the mouse. How many crossings are required to get all four creatures across the river alive

W - wolf, G - goose, M - mouth

1. Woman takes G and crosses the river.

1st bank | 2nd bank

W, M | G

2. She leaves G on the opposite second bank and turns back.

1st bank | 2nd bank

W, M | G

3. She takes W and goes with him to the bank where G already is.

1st bank | 2nd bank

M | G, W and woman

4. She takes G back to the first bank and leaves W on the second bank.

1st bank | 2nd bank

G, M | W

5. Woman leaves G on the first and takes M to the second bank.

1st bank | 2nd bank

G | W, M

6. She leaves M on the second bank and returns to the first bank.

7. Takes G and delivers it to the second bank.

1st bank | 2nd bank

| G, W, M and woman

Answer: 7 crossings.