You have two hour glasses one measure 7 minutes and other measure 4 minutes.
Using these two hour glasses, you need to measure exactly 9 minutes.
How come ?
Step1 : Start running both the hourglasses.
Step2 : when 4 minutes hourglass runs out, turn it over.
Step2 : when 7 minutes hourglass runs out, turn it over.
Step2 : Again when 4 minutes hourglass runs out, turn 7 minute hourglass over (it was running for exactly one minute.)
Step2 : 8 minute already passed and allow one minute of 7 hour glass to get over. ==>9 minute
