4 & 7 minute hourglass
easyYou have two hourglasses: one runs for exactly 4 minutes, the other for exactly 7. Neither has any markings, so you cannot read a partial time off them - you only know when a glass has just run out.
You may start them whenever you like and flip either one over the moment you choose. A flipped glass runs on whatever sand has already fallen through.
How do you measure out exactly 9 minutes?