Time Puzzle

[Lisa]

If you have two hourglasses -- one four-minute timer and one seven-minute timer, how can you measure nine minutes?

[daver676]

Flip them both over, then flip the 9 minute one over again when the four minute one runs out.

Dave

[KingFfaj]

i'd guess you'd have to turn the seven mins then turn the 4 mins, untill you think its half done?

[OttawaChris]

You watch the clock on the wall

hehe j/k

(There may be a shorter way... but I got this)
What you do is...

1) Flip both and let the 4 minute run out then stop the 7 minute... it now has 3 minutes left in it qnd the other has 4.

2) Flip the 4 and continue until the 3 minutes run out and stop the 4... this leaves 7 minutes in one and 1 minute in the other.

3) Flip the 7 and keep it going until the 1 minute is up and stop... this leaves 6 minutes and 4 minutes.

4) Flip the 4 and let the 6 run until the 4 is up... this leaves a 2 and a 4 minute.

5) Throw out the 4 minute glass

6) Let the remaining 2 minutes run out of the other glass then flip it right away and let it go for the full 7 minutes... 7+2=9

[Lisa]

Nope

[Snip3r]

count the seconds?

[Zoe]

Oooh
Okay, turn them both at the same time. When the 4 min one runs out, flip it. When the 7 min runs out flip it over too.
So then after the 4 minute one runs out again (8 minutes so far), the 7-min one has been running for one minute. So turn it over and let it run out. When it does, that will have 9 mins.

Zoe

[daver676]

OOPS! That said 7 minute, not 9. I thought that was a little too easy....

Dave

[Lisa]

Zoe got it. Yay Zoe!