You have couple of hour glasses -- A four minute hourglass and A seven minute hour glass. How can you measure 9 Minutes using them.

To time nine minutes with a four minute and seven minute timer : Start both timers simultaneously. When the four minute runs out, start it again. When the seven minute runs out, begin timing nine minute interval. Let the remaining minute fall through the four minute and run it twice more. 7-4=3 4-3=1 1+4+4=9 Simple enough.

Or another way it is as , turn over both hourglasses at once. When the four-minute hourglass runs out, turn it over. When the seven-minute hourglass runs out, turn it over. When the four-minute timer runs out this time (eight minutes have elapsed), the seven-minute hourglass has been running for one minute. Now turn over the seven minute timer back over. When the timer runs out, nine minutes have elapsed.

naming both the hour glass as 4MHG, 7MHG 0 Min - Both Starts 4 Min - 4MHG runs out, 3 left in 7MHG - Turn 4MHG 7 Min - 7MHG runs out, 1 left in 4MHG - Turn 7MHG 8 Min - 4MHG runs out, 1 reached in 7MHG - Turn both 9 Min - 7MHG runs out (Since on 1 min was left in it), 3 left in 4MHG I believe this is same as naimish's last answer

I'm late ! Anyway, congrats naimish naimish and nimesh sound so similar, I hope shabbir won't make a mistake while sending the $1

Algorithm : (1) Wait till shabbir confirms that you've won. (2) If ( You have entered PayPal Email in your profile ) ##Then : Don't worry. ##Else : Enter it ! (3) Accept the $1 from him .

I know we have a winner but here's another approach... Using a few trials work out what angle you have to hold the 4 minute one at for it to time exactly 2 minutes. Also work out some angles for other times for when some twit goes "OK smart alec, now do N minutes". Then time 7 minutes with the 7 minute timer and 2 minutes with the 4 minute timer at the appropriate angle.

Nice approach. But, I fail to understand how you measure "exactly 2 minutes" !! HOW ?! If you have a watch or something to measure "exactly" two minutes, then why go through all this at all ?? Your 7 mins hourglass can't help you too because 2/7 is not something you can "exactly" measure with naked eyes. Kindly explain your approach "more" clearly. .... To be specific, please explain how you measure "exactly" 2 mins after a few trials ?!

With a stopwatch, obviously. Which you'll need anyway, because how else would you know that the 7-minute and 4-minute timers are accurate?

@xpi0t0s In questions like these you assume that your hourglasses are correct, else by your approach, how do you know your stopwatch is correct too? you assume it to be! Or consider it that hourglasses were checked with a watch by a 3rd person and given to you... Hence you cannot have anything else.....

Oh really, that was quite cunning. We know that 'cuz he man who sets the questions defines it for us. You are not expected to use any kind of tricks here like using a stop watch. Simply use the two hourglasses, compare time as nimesh or naimish pointed out and get a measure of 9 min.