## Brainteaser 1633: New Year’s resolution

**From The Sunday Times, 26th December 1993** [link]

My New Year’s resolution is that this year I will remember my father’s birthday. To remind me I devised this formula: if I take the factorial of the number on the calendar representing the month in which he was born (1 for January to 12 for December) and multiply this by the square of the day of the month of his birth (1 to 31), the four digit result is the year when he was born.

[Note, for example, that “6 factorial” is 6×5×4×3×2×1 = 720.]

I then realised that this is no use for determining his birthday as even my own date of birth fits the formula.

Can you tell me what is my father’s date of birth?

This puzzle was included in the book *Brainteasers* (2002, edited by Victor Bryant) under the title of “Birthday resolution”. The text was changed slightly, but the puzzle remains the same.

[teaser1633]

## Jim Randell 12:41 pm

on28 July 2019 Permalink |This Python program finds day/month/year combinations that satisfy the formula, where the year is in the range [1850, 1993].

It runs in 84ms.

Run:[ @repl.it ]Solution:The father’s birthdate is 4th May 1920.There are 3 day/month combinations that give a reasonable value for the year.

These are:

So the setters birthdate must be one of the 1944 dates (18th March 1944, 9th April 1944) and his fathers birthdate must be 4th May 1920.

But it seems to me that the setter would probably be able to remember that his father was not born in 1944, so he

canuse the formula to determine his fathers birthday.The setter is obviously not a fan of

Star Wars, otherwise he would find it easy to remember his father’s birthday.LikeLike