The answer is 6.

**
This section requires Javascript.
**

You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.

After solving, i realized that i could have had solved it without doing completely . but nevermind , the solution is just using torque equation about point of contact .