Find a calculator or a pencil and paper.
Ask your friend or eveyone to write down their birthday.
Example : September 28, 1986
Ask your friend (or everyone in the room) to write down the number of the month he/she/they were born.
Example : 9 (born in September)
Multiply the month by 4
9 x 4 = 36
Add 13
36 + 13 = 49
Multiply by 25
49 x 25 = 1225
Subtract 200
1225 - 200 = 1025
Add the day of the month he/she/they were born
1025 + 28 = 1053
Multiply by 2
1053 x 2 = 2106
Subtract 40
2106 - 40 = 2066
Multiply by 50
2066 x 50 = 103300
Add the last two digits of your year of birth (1986)
103300 + 86 = 103386
Ask your friend to give you their result
103386
Now you can magically tell them their birth date.
Here's how to do it:
Subtract 10500 from their result
103386 - 10500 = 92886
9 = month of September
28 = day of birth
86 = year of birth
I hope this was the answer you were looking for. Next time, please be more clear with the query.