Hard
Math and Logic
Measure 9 mins using 4-min and 7-min hourglasses.
Choose an answer
A.Flip the 7-min twice
B.Break the glass
C.Estimate with 4-min
D.Flip 4 at 4m, 7 at 7m, 7 at 8m
Answer: Flip 4 at 4m, 7 at 7m, 7 at 8m
Run both. When 4 ends, 3 mins are left in 7. Flip 4. When 7 ends (7m total), 1 min is left in 4. Flip 7. When 4 ends (8m total), 1 min has run into 7. Flip 7 back to get that 1m.
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