If $7 x=28$ and $x+w=9$, what is the value of $x w$ ?
(A) 9
(B) 20
(C) 18
(D) 52
(E) -252
Since $7 x=28$, then $x=4$.

Since $x+w=9$ and $x=4$, then $w=5$.

Since $x=4$ and $w=5$, then $x w=20$.

ANSWER: (B)