Give the following instructions to a friend:

  1. Think of two numbers
    • the number of the month in which you were born and
    • the number of the day of the month on which you were born.
  2. Multiply the month number by 5.
  3. Add 6.
  4. Multiply by 4.
  5. Add 9.
  6. Multiply by 5.
  7. Add the Day on which you were born.
  8. Now get the final answer from your friend.

Subtract 165 to reveal the month and day that your friend was born.


Sounds familiar? It’s a nice guessing game to teach the school kids when they start doing mental math. During my school days, one of our school guests taught us a different version of the same and encouraged us to play with each other. The two numbers in our case were numbers of brothers and sisters one has. What I find ingenious about it is that these numbers were reasonably small to do the quick mental math, but at the same time not too trivial, at least in those days. I am sure this cannot be played in today’s societies, where most families have just one or two kids.

Going back to the story, after the guest left the pandemonium broke loose in the class and there were so many pairs involved in quizzing and guessing the number of brothers and sisters of each other. But, very soon almost everybody knew the number of siblings of each other.

The rules were relaxed to bring in the variety. It was ok to use even unreal numbers of brothers and sisters. The answers started turning out to be funny like 15 brothers and 18 sisters and people started having a great laugh at the imagination.  Shiv, one of the pranksters in our class soon added a punchline. After deducing these large numbers, he started exclaiming, “What! Your parents did not know about family planning?” And this led to further bursts of laughter.

Later in the day, Shiv was guessing Kiran’s numbers. Shiv rightly guessed the numbers 7 brothers and 5 sisters. He quipped, “What! your parents did not know about family planning?” But somehow the effect was unexpected to Shiv as Kiran frowned and started chasing him to beat him. The mystery was resolved later when we came to know that those were not the mock numbers but actual numbers of Kiran’s real brothers and sisters!

(featured image: Photo by Katerina Holmes from Pexels )