Ans: $k=\dfrac{5t}{m-4}$
$\begin{array}{rcl}
\dfrac{mk-t}{k+t} & = & 4 \\
mk-t & = & 4k + 4t \\
mk – 4k & = & 5t \\
k (m-4) & = & 5t \\
k & = & \dfrac{5t}{m-4}
\end{array}$
$\begin{array}{rcl}
\dfrac{mk-t}{k+t} & = & 4 \\
mk-t & = & 4k + 4t \\
mk – 4k & = & 5t \\
k (m-4) & = & 5t \\
k & = & \dfrac{5t}{m-4}
\end{array}$
