19 June 2014

Brain Drain: Puzzle after a long time

One more time (this time with a very different job) I find myself on a Thursday evening flying from DC to Atlanta. Which means time for another puzzle.

This time it is one of those about measuring arbitrary time with those dratted hourglasses. If you are reading this on FB, you probably remember that you can ask clarifying questions in the comments section but send answers by messaging me.

You have two hourglasses – one empties in four minutes and the other empties in seven minutes. You have to bake a cake but it must be baked for exactly nine minutes. What is the minimum time it will take you after I say Go! for you to bake the cake? (You don’t have to start baking the cake immediately when you turn the hourglass(es) but once you start baking, it has to finish in exactly nine minutes).

To further clarify, if you said “I will flip the four minute hourglass first and then moment it is done, I will start baking and flip both hourglasses….. And say you figured out how to bake the cake in nine minutes now using those two hourglasses”, then total time taken is thirteen minutes. Get it?


Posted June 19, 2014 by Rajib Roy in category "Puzzles


  1. By Rajib Roy on

    And the answer is 9 minutes. I am copying Ritesh ‘s answer here:
    The answer is 9 minutes. Start bith hourglasses on Go. Start baking as well. After 4 minutes, overturn the hourglass that has finished. After 7 minutes, overturn both hourglasses. The 4 minute hourglass has one minute left. At 8 minutes, when the 4 minute hourglass finishes with the 1 minute, overturn the other hourglass. It had run for 1 minute (from 7 to 8 min mark) and now has one minute left when overturned. This gets us to 9 minutes when it finishes

  2. By David Shen on

    Of course you can always use a completely different measurement tool. It takes me approximately 3 minutes to finish one beer when cooking. So by the 3rd beer, I really won’t care how long it’s been in the oven, as I will be hungry enough to eat the cake raw so then it’s all good 😀


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