You want to boil a two-minute egg. If you only have a three-minute timer (hourglass), a four-minute timer and a five-minute timer, how can you boil the egg for only two minutes?
3 minute hour glass and 5 minute hour glass is activated together After 3 minute one empties, flip the 5. Now you have exactly 2 minutes left until sand stops flowing.
Exactly how many minutes is it before seven o'clock, if 40 minutes ago it was three times as many minutes past two o'clock?