Are you the type who insists on doing absolutely everything yourself, never letting anyone else lift a finger? Could be some unresolved childhood trauma there (that’ll be 100 bucks for the session)... Now, back to our puzzle: if you do everything on your own, you’ll be delayed. Let someone else spread the secret the moment they find out, and you might just save some time.
The answer is “15”. In 5 seconds, he tells the secret to the first friend. Now two people know it. In another 5 seconds, those two each tell two more people at the same time. Now four people know it. In the last 5 seconds, those four pass it on to the remaining four.