If you have a 4-minute and a 7-minute hourglass, how do you measure 9 minutes?

Answer: Start both; when the 4 ends, flip it (4 min); when the 7 ends (at 7), flip the 4 which has 1 min of sand done — let its 1 min finish? Simpler: run the 7, then the 4 twice gives 8; combine timings to total 9.