Amelia's mom baked 48 cookies . after amelia and her friends walk through the kitchen , there are 18 cookies left . what is the percent decrease in the number of cookies ?

We assume, that the number 48 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 100% equals 48, so we can write it down as 100%=48. 
4. We know, that x% equals 18 of the output value, so we can write it down as x%=18. 
5. Now we have two simple equations:
1) 100%=48
2) x%=18
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:
100%/x%=48/18
6. Now we just have to solve the simple equation, and we will get the solution we are looking for.

7. Solution for 18 is what percent of 48

100%/x%=48/18
(100/x)*x=(48/18)*x       - we multiply both sides of the equation by x
100=2.66666666667*x       - we divide both sides of the equation by (2.66666666667) to get x
100/2.66666666667=x 
37.5=x 
x=37.5

now we have: 
18 is 37.5% of 48
minus 100 is 82.5%