[Adobe]measure 9 minutes

You are given two hourglasses. One measures 4 minutes and one measures 7 minutes. How would you measure exactly 9 minutes?


  1. we will start with both the hour-glasses and
    as soon as the sand flows to the other bulb
    we will invert it.this inversion process will be done three times for both the hourglss.
    we start measuring time after the third inversion of 4-min hourglass and measure it till the complete sand flow of 7-min hourglass.

    time= 7*3-4*3 = 9 minutes.

  2. @navin good approach.you are using total of 6 inverts.you can improve it.

  3. Time 4m timer 7 minute timer
    0 min Start Start
    4 mins Flip 3 minutes left
    7 mins 1 m left Flip
    8 mins Stop Flip (1 minute left)
    9 mins Stop

    In this approach only 3 inverts are required.


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