Painting the fence

Let the to paint the fence be $W$ , the rates of painting of Marvin and Kalngan be $m$ and $k$ per day respectively. Then $m = \dfrac W4$ and $k = \dfrac W2$ . It the number of days for two of them to paint the fence be $t$ . Then we have:

$\begin{aligned} (m+k) t & = W \\ \implies t & = \frac W{m+k} = \frac W{\frac W4 + \frac W2} = \frac 1{\frac 14 + \frac 12} = \frac 1{\frac 34} = \frac 43 = \boxed{1\frac 13} \end{aligned}$