Rotational Motion Part 2

Consider a 20kg uniform circular disc of radius 0.2m. It is pin supported at its centre and is at rest initially. The disc is acted upon by a constant force F=20N through a massless string wrapped around its periphery as shown in the figure.Suppose the disc makes n number of revolutions to attain an angular speed of 50rads−1. The value of n to the nearest integer, is .......... .
(Given, in one complete revolution, the disc