I wasn't sure whether this belonged in this section or the algebra section. Anyway, I'm studying for the math portion of the ACT and I can't remember how to solve certain problems. Like this one:

Which of the following is NOT a factor of z^5 - 16z ?

A. z^2 - 1

B. z^2 - 4

C. z + 2

D. z

E. z - 2

Now I'm aware the answer is A but I have no idea why. If someone could provide an explanation for how to solve that would be great.

Another One: If (2x - y) / (x + y) = 2/3, then x/y = ?

(2x - y is the numerator, x + y is the denominator)

F. 1/2

G. 2/3

H. 5/4

J. 5/3

K. 5

One more: For all nonzero x, y, and z such that x = yz, which of the following must be equivalent to xy?

A. z/x

B. yz^2

C. yz

D. x^2/z

E. x/y

I'm sorry if I posted in the wrong place. Thanks in advance.