Question:
Having 2 hourglasses: one 7-minute and the second one 4-minute, how can you correctly time 9 minutes?
Solution:
Turn both sand-glasses. After 4 minutes turn upside down the 4-min sand-glass.
When the 7-min sand-glass spills the last grain, turn the 7-min upside down.
Then you have 1 minute in the 4-min sand-glass left and after spilling everything,
in the 7-min sand-glass there will be 1 minute of sand down (already split). Turn the
7-min sand-glass upside down and let the 1 minute go back. And that's it. 4 + 3 + 1 + 1 = 9