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#.
