>Four people need to cross a rickety bridge at night. Unfortunately, they have only one torch and the bridge is too dangerous to cross without one. The bridge is only strong enough to support two people at a time. Not all people take the same time to cross the bridge. Times for each person: 1 min, 2 mins, 7 mins and 10 mins. What is the shortest time needed for all four of them to cross the bridge? 17 minutes, if you…
If the useful radius of the torch is 25% the bridge length, the bridge can be crossed in 15 minutes.
So the correct answer to the puzzle question is "how much of the bridge does the torch illuminate?"
Then the questioner gets flustered, and you can suggest, "Maybe you should have given them one set of night-vision goggles instead."
If you solve the puzzle as written, you can only be as clever as the puzzle-maker. If you can break the puzzle and then fix it, you must be cleverer.