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.

• Start both at the same time
  when glass of 4min'll be empty then after that start counting time of 7min glass that will be last to 3 mins. Repeat 3 times for counting 9 mins.
• 10 years agoHelpfull: Yes(2) No(0)
• 4 7
  0 3
  1 0
  7 minutes have passed
  0 6
  8 minutes have passed
  Reverse hour glass that measure 7 minutes instantly when other reach to 0
  i.e.0 1
  hence now (second hour glass will measure 1 minute more)
  Therefore,9 minutes.
  
• 8 years agoHelpfull: Yes(0) No(0)