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Problem Solving Classroom

Tips from a Master Teacher (This is a work in progress.)

Three Bulbs

Three Bulbs

There are three lights in the attic controlled by three switches on the first floor. You need to determine which of the switches controls each bulb. You are allowed to flick the switches, but you are allowed to visit the attic only once. There are no windows in the attic, and you don't have access to video equipment that you could use to monitor what's going on in the attic.

Difficulty: 2/5

Keywords: logic, light bulbs

Solution
The Solution

This problem is attacked effectively with reasoning. Define the problem. There are three bulbs but only two states: on (hot) and off (cold). We can get around this by creating a third state: warm. How do you do that? Two of the switches will be on and off, but you flip the third switch on for about 20 seconds and then turn it off. Zip up to the attic and check the bulbs. One will be on, one will be off and cold, and the third will be off but warm. Ta-da!

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