Problem of the Week 1125

Charge Your Batteries

You have eight batteries and you know that four of them are good and four are bad, but you do not know which are which. You need two good batteries for your electronic device. You can test them only by placing two batteries into the device and seeing if the device works.

How many tests are necessary before you can identify two good batteries?

Source: Brazilian Math Olympiad, 2005, Crux Math. Sept 2009, p. 293.

© Copyright 2009 Stan Wagon. Reproduced with permission.

4 November 2009