For people wondering about the hourglass problem:
1) @0min: Take both glasses (4 & 7) and flip them over.
2) @4min: When the 4min glass is empty, flip it back over
3) @7min: When the 7min glass runs out, flip it back over.
4) @8min: when the 4min glass is empty, there will be 1 minute in the 7min glass. Flip the 7 glass.
5) @9min: When the 7min glass runs out, you will be at 9min.
