This calculation lets you correctly guess a friend's birthday every time. How does it work?
Let's say that the month your friend was born is represented by a , and the day is represented by a .

By multiplying by 100, the month your friend was born is shown in the 100 (or upper) position. In other words, the month is moved to a position in front of where the birthday can be displayed (i.e. "1200" if your friend was born in December).