You have 8 batteries (4 charged batteries, 4 uncharged batteries) and a flashlight which needs 2 charged batteries to work.
You do not know which batteries are charged and which ones are uncharged. What is the least number of attempts to make the flashlight work? (An attempt consists of putting two batteries in the flashlight and checking if the flashlight works or not.)
You will need a maximum of 7 attempts to find 2 working batteries.
Break the batteries into 3 groups: two groups of 3 and one group of 2. By doing this you guarantee that at least one of the groups has 2 working batteries.
Both of the groups of 3 have 3 possible combinations of 2 batteries and the group of 2 only has 1 combination. So, 3 + 3 + 1 = 7 tries at most to find two working batteries.