You have two hourglasses: a 4-minute timer and a 7-minute timer. Without using any other measuring devices, how can you measure exactly 9 minutes?
Start both timers. When the 4-minute timer runs out, immediately flip it over. When the 7-minute timer runs out, 7 minutes have passed and 3 minutes have elapsed on the 4-minute timer. At that exact moment, flip the 4-minute timer over again to let it run out its remaining 3 minutes, adding it to the 6 minutes that pass until it hits zero? Let's trace: Start 4 and 7. At 4 min, 4 runs out, flip 4. At 7 min, 7 runs out (4 has 1 min left). Flip 4 back? No, let's keep it simple: Start both. At 4 min, 4 finishes, flip it (now 4 runs). At 7 min, 7 finishes. At this moment, 3 minutes have passed on the flipped 4-minute timer. Flip the 4-minute timer again so it runs for those same 3 minutes back to zero? No. Let's use: Start both. At 4 min, flip 4. At 7 min, 7 ends. Flip 7 immediately? Let's provide a clear concise answer.
Start both timers. When the 4-minute timer ends, flip it (it has run 4 mins). When the 7-minute timer ends, 7 minutes have passed. Immediately flip the 7-minute timer back over. When the 4-minute timer finishes its second run (at 8 minutes), stop the 7-minute timer, which has now run for 1 minute. Add that 1 minute to the 8 minutes for a total of 9 minutes.
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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