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Hourglass Problem

Having 2 sandglasses: one 7-minute and the second one 4-minute, how can you correctly time 9 minutes?

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  1. If you can wait 16 minutes then
    start 7 & 4 , reversing 4 when it is over.
    When 7 gets over, 4 will have one minute sand left.
    Use that plus twice 4 to get 9 minutes.
    Will be interesting to see it done in less time:-)

  2. first start both 7 and 4 hour glasses(hg) after 4 mins 4hg will be emptyand time is 4m
    again reverse 4 hg after 3m 7hg is empty time is 4+3=7m
    in reversed 4hg 1m sand is left also reverse 7 hg after 1m 4hg will get empty time is 7+1=8m and rev 7 hg as it has 1m sand revd for 9thmin 7
    hence calculated 9 mins

  3. take 7 & 4 do both now when we hav claculated 4 mins we hav 3 mins remaning in 7 one now reverse both after 3 mins we hav 1 min remaining in 4th one now reverse 1 mins so we hav 5mins now add 4mins to it !!!!!!!!!!!!!!!!!!!!!!!

  4. reverse both the sand glasses at a time, so that we get 3 mins.if we do this for 3 times we get 3+3+3=9