You have 8 batteries, numbered 1 through 8. Four of them are good, and four are bad. But you don't know which are which.
You have a flashlight that requires 4 batteries to work. What is the minimum number of times you need to put in the batteries to guarantee to get the flashlight working?
Assume that the flashlight either works (when all four are correct) or not (for any other combination). Nothing in between. And the total number of times includes the final time you put them in, even if at that point you know you have a workable set.