How do i find 45% of 320 by multiplying?

1. We assume, that the number 320 is 100% - because it's the output value of the task. 
2. We assume, that x is the value we are looking for. 
3. If 320 is 100%, so we can write it down as 320=100%. 
4. We know, that x is 45% of the output value, so we can write it down as x=45%. 
5. Now we have two simple equations:
1) 320=100%
2) x=45%
where left sides of both of them have the same units, and both right sides have the same units, so we can do something like that:
320/x=100%/45%
6. Now we just have to solve the simple equation, and we will get the solution we are looking for.

7. Solution for what is 45% of 320

320/x=100/45
(320/x)*x=(100/45)*x       - we multiply both sides of the equation by x
320=2.22222222222*x       - we divide both sides of the equation by (2.22222222222) to get x
320/2.22222222222=x 
144=x 
x=144

now we have: 
45% of 320=144

Well the percent sign, %, literally means out of 100. So, you are multiplying like this:
.45 • 320
=
144