Take the day number of your birthday (eg. If your birthday is 28 April, take 28). Multiply your number by 4. Add 14. Multiply the result by 25. Subtract 230.

Take the day number of your birthday (eg. If your birthday is 28 April, take 28). Multiply your number by 4. Add 14. Multiply the result by 25. Subtract 230. Add the month number of your birthday (ie. May = 5, November = 11) Subtract 120. Following the steps above works out your birthday. For example, if your birthday is 19 August the result of the formula is 1908. Using algebra, show how this trick works. (Hint: Let the day number = d and the month number = m)

$\begin{array}{rcll} ( d \cdot 4 + 14 ) \cdot 25 - 230 + m - 120 \\ &=& ( d \cdot 4 + 14 ) \cdot 25 - 350 + m \\ &=& d \cdot 4\cdot 25 + 14 \cdot 25 - 350 + m \\ &=& d \cdot 100 + 350 - 350 + m \\ &=& d \cdot 100 + m \\ \end{array}$