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.

Similar Riddles

Question: What tastes better than it smells?

Question: What goes in the water black and comes out red?

Question: What loses its head in the morning and gets it back at night?

Question: What two things can you never eat for breakfast?

Question: Your father is a scientist who has invented a red pill which, if eaten with 1 blue pill which he has invented, will grant immortality. The night he invents it, he gives you 2 red and 2 blue pills just in case one of them is lost or substandard. He also warns you that an overdose will cause the opposite effect and kill you instead. You put the pills in your pocket and leave his lab for home. On the way home, you are abducted by aliens who blindfold you and throw you into a singularity. At this point, you remember the pills your father gave you. You take them out (you can move and have enough oxygen in space for a short time), but realize that you can't tell the red pill from the blue pill. Even if you take off your blindfold, you can't see anything due to your proximity to the black hole. Given the circumstances, how do you successfully eat 1 red and 1 blue pill and survive?