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.