What is the birthday numbers game?

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In E.L. Doctorow's novel *Billy Bathgate*, the character of Abbadabba Berman is that of a man who is extremely gifted in Math and calculations. He is described as a person that "cannot not help himself" to think of anything but numbers. He devises a way to calculate numbers linearly so that, in whatever direction you add them, they can add up to the same sum. In other words, Mr. Berman is a genius whose intelligence gets the best of him.

One of the "games" that Abbadabba creates is to guess a person's date of birth based on a formula that he comes up with. He first asks Billy to give a number to each month of the year, for example, January is 1, February is 2, etc.

After that, Billy has to think of his original birthday month and not tell him when it is. Then he has to take the number of the month that comes after his own birthday month, take the sum, multiply it by 5, and then by 10. To that product he was to add the number of the day when he was born. In Billy's case the final number he comes up with is 959. Hence, when you separate the digits, you get that Billy's birthday month was the ninth month and the ninth day-September 9th.

For a date such as 7/31/1992, for example, you would have to take the number of your month (7) and add the number of the next month (8)= 15. Then you have to multiply that 15 times 5=(75) and then the 75 you would multiply times 10 (750). If the day you were born is the 31st, then you have to add 750+31 (781). However, you really do not add the digits, but skipthe middle digits and use the first and the last in the sum of 7 (isolate it) and then Abbadabba would have known that the number you added was 31 to make the total of 781.

That's how you would follow this extremely complicated route to answer a very simple question.

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