Answer:
B
Step-by-step explanation:
rotate 180 --> translate up by 4: (x,y)--> (-x,-y) --> (-x,-y +4)
The numbers of new CDs is 30 and the old CDs is 21.
To solve this problem, we have to write a system of equations in which we would know the numbers of each types of CD she bought.
<h3>System of Equations</h3>
This is used to solve a series or complex of word problems in which we can represent in a mathematical statements;
- Let n represents numbers of new CDs
- Let u represents numbers of old CDs
The equations can be represented as

Let's take equation (i)
Making n the subject of formula

we can put equation (iii) into equation (ii)

Let's put the value of u into equation (i)

From the calculations above, the numbers of new CDs is 30 and the old CDs is 21.
Learn more on system of equations here;
brainly.com/question/13729904
Add them up and you have your answer
Your answer would be 4, because the equation for the circumference of a circle is π × diameter, so if you have 4 × π as the circumference, then 4 is the diameter.
I hope this helps!