Two ends A and B of a smooth chain of mass m and length l are situated as shown in figure. If an external agent pulls A till it comes to same level of B, work done by external agent is : 

\(\frac{mgl}{36}\)

Final figure is shown below. It can be obtained as follows.

We can assume that a part of length \(\frac{l}{6}\) is cut from the lower portion of side B, and put below A.

Mass of this part \( = \frac{m}{6}\)

This part rises by \( = \frac{l}{6}\)

Work done = Change in potential energy

\(\frac{m}{6}g\frac{l}{6} = \frac{mgl}{36}\)