Correct answers are:
• Find another bridge across the river.
• Throw two pins (or all of them) across the river to the other bank, and then walk across.
• Tie a rope to the pins, then float them across the river while walking across the bridge.
• Fasten the pins to his body, then swim across the river.
I've never seen this puzzle in physics textbooks, but it relates to the following example, which is.