4 MINUTE TIMERS HOW TO
Not sure how to rate this for difficulty. At this point it will have been running for 1 minute, we will flip 1 minute's worth of sand back to the top and it will run for another 1 minute.
Since the twice-flipped 4 minute hourglass will stop at 8 minutes we can flip our 7 minute hour glass here for the second time. We need to add 2 minutes on to the end of the 7 minute hourglass. And there is a solution that allows this. As shown:Ĭlearly though a better solution would be one that didn't require us to wait 7 minutes before we start. We just then need to flip it two more times to get 9 minutes. We said if we start them both together and flip the 4 minute hourglass every time it runs out the difference between the 7 minute hourglass ending and the and the 4 minute hourglass ending for the second time is 1 minute.
It's most of the way there in the 'Difference' section above. If we flip it now it will necessarily run for 4 minutes. When the 4 minute hourglass runs out we know that the 7 minute hourglass has run for 4 minutes also and so must have 4 minutes of sand in the bottom. We could for example start both timers together. The difference between 4 runs of the 4 and 3 runs of the 7 would be 16 as against 21 respectively. If we were to start both hourglasses together, flip the 4 when it reaches the end the difference between it ending for the second time and the 7 minute ending is 1 minute.
We could use the 4 and the 7 one after the other to measure 11 minutes. Before we can take a run at it we're going to have to build some tools, we will then use them in combination. It's not dissimilar to the 'Die Hard Water Puzzle' but it is somewhat more complicated. Before reading the answer can I interest you in a clue?
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