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Hard boiled eggs! Maths based quiz question...
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You have one 7 minute egg timer and one 4 minute egg timer. How do you accurately measure 9 minutes without taking any longer than 9 mins?
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Yes, timing it on your iPhone seems like a perfectly sensible way to do it. If you are stuck with only egg timers, though:
Start both egg timers. When the 4 runs out, turn it over (4 minutes gone). When the 7 runs out, turn it over (7 minutes gone). When the 4 runs out this time (8 minutes gone), the 7 has been running for 1 minute. Turn it over once again (1 minute of sand is now on top). When it stops, 9 minutes are up.
Start both egg timers. When the 4 runs out, turn it over (4 minutes gone). When the 7 runs out, turn it over (7 minutes gone). When the 4 runs out this time (8 minutes gone), the 7 has been running for 1 minute. Turn it over once again (1 minute of sand is now on top). When it stops, 9 minutes are up.
Turn both at the same time
When the 4 min one has elapsed the other one will be split 3 and 4 minutes
turn them again
when the 3 minutes (in the 7m timer) has elapsed the 4 m one will be split 3 and 1 and the other 7 and 0
Turn them again when the 1 min has elapsed turn the other one back over for the final minute
When the 4 min one has elapsed the other one will be split 3 and 4 minutes
turn them again
when the 3 minutes (in the 7m timer) has elapsed the 4 m one will be split 3 and 1 and the other 7 and 0
Turn them again when the 1 min has elapsed turn the other one back over for the final minute
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