The way I am going to solve this is through fractions.

As we should all know, #x% = x/100#

this means that we can make this equation:

#18/80 = x/100#

We can solve this by finding out how #18/80# gets to #x/100#

#(18 xx y = x)/(80 xx y = 100#

In this equation, it is important to remind you that #y# in all parts of this equation are the same.

This means that you need to multiply #80# to get to #100# by the same number as you need to multiply #80# to get to #x#. Now we can solve the equation.

First, we find out what #y# is from simple algebra working on the denominator of the equation.

# 80 xx y = 100#

We can move #80# to the other side by dividing each side by #80#

#cancel(80)/cancel(80) xx y = 100/80#

#y = 100 ÷ 80#

#color(lime)(y = 1.25#

Now we can input #1.25# in for #y#.

#(18 xx y = x)/(80 xx y = 100#

#(18 xx 1.25 = x)/(80 xx 1.25 = 100#

#18 xx 1.25 = x#

Now we can use simple math to find #x#.

#18 xx 1.25 = x#

#18 xx 1.25 = 22.5#

#color(blue)(x = 22.5#

Now we can input #x# for #22.5# in the original equation.

#18/80 = x/100#

#18/80 = 22.5/100#

#therefore# #18# is #22.5%# of #80#.