Guess Your Birthday

Enter the number 7
Multiply by the month of your birth
Subtract 1
Multiply by 13
Add the day of your birth
Add 3
Multiply by 11
Subtract the month of your birth
Subtract the day of your birth
Divide by 10
Add 11
Divide by 100

And, amazingly, your friend's birthday will appear on the calculator screen! For example, let's suppose your friend was born on February 29. Below, I repeat the sequence of instructions, showing the result at each step in red.

Enter the number 7 (7)
Multiply by the month of your birth (7x2=14)
Subtract 1 (14-1=13)
Multiply by 13 (13x13=169)
Add the day of your birth (169+29=198)
Add 3 (198+3=201)
Multiply by 11 (201x11=2211)
Subtract the month of your birth (2211-2=2209)
Subtract the day of your birth (2209-29=2180)
Divide by 10 (2180/10=218)
Add 11 (218+11=229)
Divide by 100 (229/100 = 2.29)

This will work every time! Try it!