Birthday Trick (European format)
from Shekhar Dutta
West Bengal, India
Ask your friend to do these calculations.
An example for 13th August, 1999 is shown below each step.
Multiply his/her date of birth by 20. (only date)
13*20=260
Add 222.
260 + 222 = 482
Multiply by 5.
482*5=2410
Add the birth month.
2410 + 8=2418
Multiply by 100.
2418*100=241800
Add the last two digits of the birth year.
241800 + 99=241899
Add 111.
241899 + 111=242010
Ask him/her the answer and you will be able to say his/her date of birth.
Trick - Subtract 111111 (six 1s) from the answer.
242010-111111=130899
Separate the digits by twos.
13/08/99
That is the birth date (in the European format).
13th August, 1999.
Explanation of the Birthday Trick
Let D = Day born
Let M = Month born
Let Y = last two digits of the birth year
Multiply his/her date of birth by 20.
20*D
Add 222.
20D + 222
Multiply by 5.
5 (20D + 222) = 100D + 1110
Add the birth month.
100D + 1110 + M
Multiply by 100.
10000D + 111000 + 100M
Add the last two digits of the birth year.
10000D + 100M + YY + 111000
Add 111.
10000D + 100M + YY + 111111
Trick - Subtract 111111 from the answer:
10000D + 100M + YY
Note: This trick gives you the date in the European form: DD/MM/YY
Ask your students to do the following:
Change this trick so that the answer comes out in the American format: MM/DD/YY
Here is the solution:
Let D = Day born
Let M = Month born
Let Y = last two digits of the birth year
Multiply his/her birth month by 20.
20*M
Add 222.
20M + 222
Multiply by 5.
5 (20M + 222) = 100M + 1110
Add the birth day.
100M + 1110 + D
Multiply by 100.
10000M + 111000 + 100D
Add the last two digits of the birth year.
10000M + 100D + YY + 111000
Add 111.
10000M + 100D + YY + 111111
Trick - Subtract 111111 from the answer:
10000M + 100D + YY