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Hour glass problem

(11 votes, average: 4.27 out of 5)

June 19, 2017 by Neha Waghmare 3 Comments

You are given two hour glass which can measure 4 and 7 min respectively. now you have to calculate 9 min using these two hour glass (Note: you have to calculate it at a stretch)

Click here to see SolutionHide
lets say we have two hour glasses A and B and let’s keep there top and bottom time as (t,b) for our understanding.

so initially
A: (4,0) B: (7,0)

reverse A after it is over (i.e after 4 minutes)

A: (4,0) B: (3,4)

reverse B after it is over (i.e after 3 minutes)

A: (1,3) B: (7,0)

reverse A after it is over (i.e. after 1 minute)

A: (4,0) B: (6,1)

After A is over (i.e after 4 minutes)

A: (0,4) B:(2,5) —
Start your timer now and count 2 minutes from B hour glass and than reverse it after it is over and count 7 more minutes from it. thus you can count a total of 9 minutes at a stretch.

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Comments

  1. Vishal Singh Chauhan says

    July 17, 2017 at 12:01 am

    1.Start both hour glass at the same time…
    When the 4 min completes in 4min. Hour glass.. The 7min hour glass has still 3 minutes left..

    2. Reverse the 4 min hour glass…and start comparing it with the 7 min hour glass… when the 3 mins completes which was left in 7min hour glass..the 4 min Hour glass has still 1 minutes left..
    Start Counting the time from that moment…

    (so we will get 1 minute in this way)

    3. When the 1 min. Completes reverse the 4 min hour glass again… And when thus time the 4 min hour glass completes..

    (U now have counted 5 mins in this way)

    1min.+4min. =5min.

    4. Now reverse the 4min hour glass again… And when it completes…

    U now have counted 9 minutes.

    1min. +4min.+4min. =9min.

    Reply
  2. SasiKanth Jam says

    June 20, 2017 at 12:58 am

    Start both 7 min & 4 min hour glass at the same time.

    once 4 hour glass completes then reverse it, now 7 hour glass is at 4 min and 3 min remaining..
    when 7 hour glass completes then reverse it. the reversed 4 hour glass still has 1 min to complete..
    after you reverse 7 hour glass wait unitl the 4 hour glass to be completed thereby your total is 9 min.

    Reply
  3. irnathhingmire says

    June 19, 2017 at 1:29 pm

    Start both glass at same time.

    At 4 hour:
    When 4 hour glass reaches its limit reverse it.
    both glasees will be at 4 hour. The 7 hour glass remains 3 hour sand.

    at 7 hour:
    As soon as 7 hour glass emptied reverse it.
    The sand remains in 4 hour glass is for 1 hour.
    So both glass will run for another one hour.

    At 8 hour:
    now 4 hour glass will going to be emptied. As soon as it emptied it shows 8 hour. And the sand fall in 7 hour glass will also be of 1 hour.
    Reverse the 7 hour glass immediately…it will run for another one hour. Thua we can count 9 th hour.

    Reply

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