Physics Beginner!

First, ask yourself what shape is the poster and what is the area of that shape?

 

You don't need to know the actual area of the poster, but just how you would calculate the area. What is the formula?

Now if you reduced the length and the width each by 15%, how would the new length and width compare with the original values? What would you find for the new area?

 

Ah, you can not do it like that. Example, Let's suppose length = 100. Then 100-15%=100-.15= 99.85 which is not equal to 85%(i.e. 15% off) of 100 which is 85.

If you do it like that then It is L-.15*(blank). What is blank? From then, you can simplify it further.

 

It's some variable kind of like x or like a

If you are taking 15% off, then L-.15*x. What is x? Then simplify this expression.

 

I'll make it simple. L-.15L = L(1-.15) .85L (which can be interpreted as 85% of L). This can be done with W as well. After you've done that, find the new area in terms of L and W. Then you should be able to compute the percentage off from the original area.

 

I was just using x to test what you've already know about percentage. In this case, x=L. If you are taking a percentage of a number, so you must convert that percentage into a decimal and multiply by that number. Since you are taking 15% off, then you must subtract from the original number
i.e. number - .15*number which is the same as .85* number

 

No, you can not assume that L=W. You don't need to know the numerical value... for the area nor the width or length. You are figuring out the percentage change in area.
Old Area = LxW.
Okay, good. Since you've now know that the New Area=(.85L)(.85W), simplify this equation. Use basic algebra. Then, use the definition of a percentage change and find it.

i.e. [tex]change_{area} = \frac{Area_{new}-Area_{old}}{Area_{old}}*100[/tex]

I'm sleepy. You can figure this out by yourself. It's straightforward.