**Problem Solution****Hourglass****Having 2 sandglasses: one 7-minute and the second one 4-minute, how can you correctly time 9 minutes?**

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Solution:

T = 0min ---- Turn over the 4 and 7 minutes glasses.

T = 4min ---- The 4 runs out, Flip it. (3 minutes in 7 glass)

T = 7min ---- The 7 runs out, Flip it. (1 minute in the 4 glass)

T = 8min ---- The 4 runs out. 1 minute in bottom of 7 glass, Flip 7 glass.

T = 9min ---- The 7 runs out, 9 minutes reached +/- about 5 seconds to allow for flipping time.