Everybody has to cross the river.

This is what you do:

There are 17crossings involved.

• Policeman and Thief
• Policeman
• Policeman and Son
• Policeman and Thief
• Father and Son
• Father
• Father and Mother
• Mother
• Policeman and Thief
• Father
• Father and Mother
• Mother
• Mother and Daughter
• Policeman and Thief
• Policeman and Daughter
• Policeman
• Policeman and Thief

Well, OK, if you must be pedantic, you could substitute Mother for Father and Daughter for Son - just so long as you substitute all instances, natch..