You have a flashlight and 8 batteries

Question: You have a flashlight that takes 2 working batteries. You have 8 batteries but only 4 of them work.

What is the fewest number of pairs you need to test to guarantee you can get the flashlight on?

Riddle Discussion

By: 0wewewe0 on 13/9/14

7. Let's name batteries ABCDEFGH. First let's test AB, CD, EF, GH (4 tests). in worst possible case neither on those pairs will work but we now know for sure that each pair has at least one working battery. Now we take AB and CD and try different combinations of them : AC, BC, AD, BD. In worst possible case the first three(3 tests) dont work and now we know for sure that the forth must work.

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