Here is a Math Formula trick to have fun with your friends, to guess their Month of Birthday given their NRIC, within two tries.
(only works for Singapore citizens born after 1970)
The formula is: take the 3rd and 4th digit of the NRIC, put them together, divide by 10, and multiply by 3.
For an example, if a person’s NRIC is S8804xxxx, we take 04, divide by 10 to get 0.4
Then, 0.4 multiplied by 3 gives 1.2
Then, guess that the person is either born in January (round down 1.2 to 1) or February (round up 1.2 to 2). There is a high chance that you are right! Usually, round up for the first six months (Jan to Jun), and round down for the last six months (Jul to Dec).
This formula was developed and tested by me. There are some exceptions to the rule, but generally it works fine especially for people born from 1980 to 2000.
Hope you have fun with maths, and impress your friends!