Impossible Riddle?????

Question

What's the answer to the riddle on

http://www.mathparadise.com/?p=41

Please tell me, thanks!

Answer 1

Start both hourglasses together. When the 4-minute glass runs out, turn it over (4 minutes have elapsed). Then the 7-minute glass runs out, turn it over (7 minutes have elapsed). When the 4-minute glass runs out this time (8 minutes have elapsed), the 7-minute glass has been running for 1 minute. Turn it over, when it stops, 9 minutes have elapsed.

Answer 2

1. (time elapsed: 0 min) Start both hourglasses at the same time

2. (time elapsed: 4 min) 4 min hourglass runs out

    Restart 4 min hourglass

3. (time elapsed: 7 min) 7 min hourglass runs out

    Restart 7 min hourglass

4. (time elapsed: 8 min) 4 min hourglass runs out

    RESTART 7 MIN HOURGLASS

5. (time elapsed: 9 min) 7 min hourglass runs out

Explaination:

When the 4 min hourglass runs out the first time, the 7 min hourglass still has 3 minutes to go.

Therefore, when the 7 min hourglass runs out for the first time, the 4 min hourglass has 1 min to go.

TIME NOW: 7 MIN

When the 4 min hourglass runs out, the 7 min hourglass has only completed 1 min of timing.  It is restarted at that time

TIME NOW: 8 MIN

With 1 min of sand left on the top, the 7 min hourglass would run out after one min

TIME NOW: 9 MIN

Answer 3

use the 7 minute hourglass first, then use half of the four minute hourglass 7 + 2 = 9 minutes.

Answer 4

Start both hourglasses. When the 4-minute glass runs out, turn it over (4 minutes elapsed). When the 7-minute glass runs out, turn it over (7 minutes elapsed). When the 4-minute glass runs out this time (8 minuts elapsed), the 7-minute glass has been running for 1 minute. Turn it over once again. When it stops, 9 minutes have elapsed.  

Answer 5

okay, here we go...

Firstly you have to start both hourglasses at the same time

When 4 minutes is up, turn over the 4 minute one again.

When the 7 minute hourglass runs out turn it over again.

Now, when the other hourglass (4minute) has run out, you have 8 minutes from the 4 minute one twice.  

At this time the 7 minute one has been running for 1 minute.

8 minutes and 1 minute = 9 minutes!

hope this has helps

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