答案：B
$\begin{array}{rcl}
\dfrac{\text{混合茶葉的總成本}}{\text{混合茶葉的總重量}} & = & \text{每公斤混合茶葉的成本} \\
\dfrac{140x + 315y}{x + y} & = & 210 \\
140x + 315y & = & 210x + 210y \\
105y & = & 70x \\
\dfrac{x}{y} & = & \dfrac{2}{3} \\
x : y & = & 2 : 3
\end{array}$
$\begin{array}{rcl}
\dfrac{\text{混合茶葉的總成本}}{\text{混合茶葉的總重量}} & = & \text{每公斤混合茶葉的成本} \\
\dfrac{140x + 315y}{x + y} & = & 210 \\
140x + 315y & = & 210x + 210y \\
105y & = & 70x \\
\dfrac{x}{y} & = & \dfrac{2}{3} \\
x : y & = & 2 : 3
\end{array}$
