You have 15 balls, 2 of them are radioactive. You have to run 7 tests (no more) on the balls which will sort out the 2 radioactive ones guarenteed every time. You can test them in groups, or even one by one.
A "test" comprises of taking a subset of the balls and checking if this subset contains a radioactive ball (one or more) or not. The test does not determine the number of radioactive balls in the tested subset.