If $x=2 y$ and $y \neq 0$, then $(x+2 y)-(2 x+y)$ equals
(A) $-2 y$
(B) $-y$
(C) 0
(D) $y$
(E) $2 y$
We simplify first, then substitute $x=2 y$ :

$$
(x+2 y)-(2 x+y)=x+2 y-2 x-y=y-x=y-2 y=-y
$$

Alternatively, we could substitute first, then simplify:

$$
(x+2 y)-(2 x+y)=(2 y+2 y)-(2(2 y)+y)=4 y-5 y=-y
$$

ANSWER: (B)

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