Good Puzzles

1) “Using only a four-minute hourglass and a seven-minute hourglass, measure exactly nine minutes–without the process taking longer than nine minutes,”

Ans: i) Start the both hourglass
       ii) After 4 minutes, reverse the both hourglass: 4-min glass start from 4 and 7-min glass start from 3
      iii) After 3 minutes, reverse the both hourglass: 4-min glass start from 1 and 7-min glass start from 7
      iv) After 1 minute, reverse the both hourglass: 4-min glass start from 4 and 7-min glass start from 1
      v)  After 1 minute, stop.
                Total measurement= 4+3+1+1=9 minutes