Posted by **Rebekah** on Tuesday, December 13, 2011 at 3:06am.

You have a 5 minute hour glass and an 8 minute hour glass. You need to cook something for exactly 9 minutes. How will you use the two hour glasses to determine the exact time? There is no way to determine where the halfway mark is with the hourglasses.

Help me please!

- Math for Liberal Arts -
**bobpursley**, Tuesday, December 13, 2011 at 8:08am
What is 3*8-3*5?

- Math for Liberal Arts -
**Steve**, Tuesday, December 13, 2011 at 12:19pm
Not quite sure how to subtract times using an hourglass. Can't drain the 8' one and then subtract the 5' one.

However, if we do the following, I think it works. There may be more elegant ways:

The three columns show the running time, and the operations of turning glasses, with their remaining sand

time 8' 5'

0 8 5

5 3 0

turn 5'

8 0 2

turn 8'

10 6 0

turn 5'

15 1 0

Now you can start cooking, with 1' of sand in the 8' glass.

After 1 minute, turn the 8' glass and let it drain.

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