A block which has a mass of $4\,kg$ is resting over a book stand as seen in the diagram from below. This block is tied to another which has a mass of $2\,kg$ by a thin wire which goes through a rugged pulley. This pulley is formed by a disk which has a radius of $50\,cm$ and has a moment of inertia equal to $1\,kg\cdot m^2$. Given these conditions find the time in seconds which will take the block which has $2\,kg$ to descend $4\,m$.

The alternatives given are as follows:

$\begin{array}{ll}

1.&\textrm{1 second}\\

2.&\textrm{2 seconds}\\

3.&\textrm{3 seconds}\\

4.&\textrm{4 seconds}\\

5.&\textrm{5 seconds}\\

\end{array}$

How exactly can I tackle this problem?. I'm not very sure how to use the formula for the moment of inertia to find the time. Could it be that this is related with the angular acceleration for the pulley?. The only formula which I do recall to estimate the time when a body is hanging is given by:

$v_f^2=v_o^2-2g\Delta y$

But I don't know exactly if this can be related to this problem. Can someone help me with the right approach for this problem?.