Let's go through this problem step by step while bearing in mind the concept of
PEMDAS.
Step 1:

Declaration of the expression. Nothing wrong here yet.
Step 2:

Clarise evaluated what's inside the parenthesis first - which was the right thing to do! There are no mistakes in this step.
Step 3:

In this step she subtracted first. This should not be the case! PEMDAS tells us that the exponents and division gets higher priority than subtraction. This is therefore the first mistake Clarise makes.
Step 4:

In this step Clarise evaluates the exponent. This does not violate any rules (relative to the previous expression) since PEMDAS tells us that exponents take higher priority than division.
Step 5:

(Clarise's final answer)
In Clarise's final step, she manages to get the wrong answer! Dividing 51.68 by 0.16 would give us 323. This is another mistake of Clarise.
Looking at the choices, we can now identify what mistakes Clarise made:
-She subtracted before evaluating the exponents
-She subtracted before she divided
-She divided incorrectly
If you're talking about the books in between, it will/be 20 books
You need to give us the equation first, otherwise we can't do it
:|
Answer:
l = 16, w = 8
Step-by-step explanation:
Let l = length
Let w = width
Area = w * l
128 = w * l
w = 128/l
Perimeter = 2w + 2l
48 = 2w + 2l
48 = 2(128/l) +2l
48 = 256/l +2l
Multiply everything by l
48l = 256 + 2l^2
l^2 + 128 = 24l
l^2 + 128 -24l = 0
(l-16)(l-8)= 0
L = 16
L = 8
If you try them out, and test it, l = 16 would work:
2(16) + 256/16 = 48
32 + 16 =48
16 * 128/16 = 128
16 * 8= 128
128 = 128
Since I'm not sure if you are in advanced math or not, I will do it simple with guess and check
Try w is