"Percent" literally means "per #100#". If we say something is #18%#, that is the same as writing #18/100#. When we say #18%# of a number, it is similar to how we might say #3# of a number. In the latter case, we would multiply by #3# to represent it algebraically. In this case, we multiply by #18% = 18/100#.

Let #x# represent our number. Writing our given information algebraically, then, we get

#18/100x = 9#

#=> 18/100x * 100/18 = 9*100/18#

(we multiply by the reciprocal of the coefficient of #x# to make #1# the new coefficient of #x#)

#:. x = 50#