Solve the equation for the letter d: c equal pi times d a. d = c minus pi b. d equal c times pi c. d = c over pi d. none of the above

Remark
You are starting out with an equation that has no addition or subtraction associated with it. Therefore, you cannot use addition of subtraction to isolate a variable, which makes A wrong. You can only use multiplication or division to isolate a factor. 

Argument
A
A is wrong. See above.

B
B is wrong as well. The reason is very important to know in algebra. Let's consider what you are given and what you want.

C = pi * d is what you are given.
Pi and d are multiplied together. If you want to have d all by itself, the only way to do it is to do the opposite of multiplication. That's division. 

You divide both sides by what is with the factor you want to have alone.  You want d. You have to divide by pi. What you get looks like this.



So b is wrong.

C
is correct. See the discussion under B