Hour Glass Puzzle
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I have two sand hourglasses:
1. A 7 minute one and
2. An 11-minute one.
Using just these 2 sand hourglasses, how can I measure time as 15 minutes? 
Solution 1: (If you DON’T have to start measuring immediately) Start both hourglasses together. When the 7-min one runs out, start measuring. At that time, the 11-min one will have 4 min. left. After the 11-min one runs out (i.e., 4 min. from starting the measurement), invert it and run it till it runs out (11 min.), completing 15 min.
Solution 2: (If immediate start is required) Start both the hourglasses together and start measuring. When the 7-min one runs out, invert. At that moment, the 11-min. one will have 4 min. left. Let the 7-min. glass run till the 11-min. one runs out. (Time since start = 11 min.) By this time, the 7-min. glass will have run it for 4 min. since it was inverted. Invert it once again and let it run till it runs out, adding 4 more min., completing 15 min.
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