Ans: B
$\begin{array}{rcl}
\dfrac{2m-n}{m-2n} & = & 3 \\
2m-n & = & 3(m-2n) \\
2m-n & = & 3m-6n \\
m & = & 5n \\
\dfrac{m}{n} & = & \dfrac{5}{1} \\
m:n & = & 5:1
\end{array}$
$\begin{array}{rcl}
\dfrac{2m-n}{m-2n} & = & 3 \\
2m-n & = & 3(m-2n) \\
2m-n & = & 3m-6n \\
m & = & 5n \\
\dfrac{m}{n} & = & \dfrac{5}{1} \\
m:n & = & 5:1
\end{array}$
