Answer:
Part A) see the explanation
Part B) 
Step-by-step explanation:
Part A)
we know that
A relationship between two variables, x, and C, represent a proportional variation if it can be expressed in the form
or 
Let
C ---> is the total cost (represent the output or dependent variable)
x ----> is the number of books (represent the input or independent variable)
In this problem
we have a proportional relationship between the variables x and C
Part B) we know that
The linear equation is given by

The constant of proportionality k is the same that the slope or unit rate of the linear equation
In this problem the constant k is given

so

For x=12 books
substitute in the equation and solve for C

In function notation

Answer:
the equation to use would be D (255-20(5) = ?). To solve, first multiply 20 and 5 and you would get 100 so the equation would be 255 - 100 = ? Then you would subtract 100 from the 255 to get $155 left. To solve for 7 weeks you would replace the 5 in the equation with 7 so your equation would be 255 - 20(7) = ? Next you would multiply 20 and 7 to get 140. the equation would then be 255 - 140 = ? Subtract 140 from 255 and you would get 115. So after 7 weeks she would have $115 left in her account.
The

confidence interval of a standard deviation is given by:

Given a sample size of 31 charge sales, the degree of freedom is 30 and for 90% confidence interval,

Therefore, the 90% confidence interval for the standard deviation is given by
=<span><span><span><span><span><span>14<span>x^6</span></span>+<span>9<span>x^5</span></span></span>+<span>4<span>x^4</span></span></span>−<span>14<span>x^3</span></span></span>+<span>12<span>x^2</span></span></span>+<span>11</span></span>