Where Did The 100 Come From?

About a month ago I posted this tweet on my stanleyinparis' account:  

But where did the number 100 come from ?

Simple a person can write the formula this way: 

X%  of  30 = 19.5

The percent sign "%" just by itself has a value of  1/100, so we simply replace the percent sign with the value of 1/100.

X (1/100) of 30 =19.5

X/100 of 30 =19.5 

30X/100 = 19.5