Solution #28: Year of Birth
Let T, h, t and u be the four digits that make up your year of birth
(Thousands, hundreds, tens, and units).
Your hear of birth can be written:
1000T + 100h + 10t + u.
If you take away
T + h + t + u
you are left with
999T + 99h + 9t
which is divisible by 9.
Source: Berrondom, Marie
Categories: Reasoning, Number sense, Arithmetic, Magic