Today Puzzle #553
Puzzle No. 553鈥� Monday 26 August
If I have an 11 second sand timer and a 7 second sand timer, how can I time 15 seconds exactly? The 15 seconds must start when you start the first timer.
Today’s #PuzzleForToday has been set by David Feather a retired maths education lecturer
Click here for the answer
Start them together. Restart the 7 second timer after 7 seconds. When the 11 seconds is up invert the 7 second timer which will have run for 4 seconds and so will take a further 4 seconds to make 15.
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