# Two sand glasses – Brain Teaser

What is the quickest way to time a 9 minute cooking session with two sand-glasses of 4 and 7 minutes respectively?

**Post your answer and calculations below.**

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A riddle to challenge your mind | **Back – Next** | Arrange the numbers 1-9

Tell your cook to do it ðŸ™‚

At Time=0

Start both 4 & 7 Timer

At Time= 4

Restart The (4 min.) Timer

At Time = 7

Restart The (7 Min.) Timer

** Begin The Cooking

At Time = 8

(When The 7 Min. timer run out)

restart the 4 min. timer

At time= 12

restart the 4 min. timer

So : the cooking time = 1 + 4+ 4 = 9 min.

Good job everyone,

Here is my solution. It is fairly similar to some of yours.

Start both timers simultaneously, then turn the 4 minute one over as soon as it runs out. Then when the 7 minute timer runs out, turn it over. When the 4 minute timer runs out for the second time, turn over the 7 minute timer which has been running for 1 minute, and when it runs out, hey presto 9 minutes have elapsed.

Thanks.

Ken.