This is a math trick that General Douglas MacArthur performed to entertain people. Ask your friend to write down his or her birth month and age, both as an integer. For example, if someone was born in July and the age is 18, write down 7 and 18. Then ask your friend to do the following calculation.
Month of birth: 7
Double it: 2 * 7 = 14
Add 5: 14 + 5 = 19
Multiply by 50: 19 * 50 = 950
Add age: 950 + 18 = 968
Subtract 365: 968 - 365 = 603
Ask him to call out the result when he has it. Then you secretly add 115 to the number, which will be 603 + 115 = 718. Aha, you got his birth month and age. The first digit (or the first two if it is a four-digit number) indicates his birth month and the last two digits indicate his age. Try it with a different friend to see if it still works!