3 and 4 th ques plzzz with proper sol ..from meritnation experts 
 

Dear Student ,
Moment of inertia of a disc about an axis passing through its centre and perpendicular to its plane is ,
$I=\frac{1}{2}m{r}^2$
Now when the radius r' = 2r then ,moment of inertia becomes ,
$I\text{'}=\frac{1}{2}m{\left(2r\right)}^2=\frac{1}{2}m\times 4r^2=4\times \frac{1}{2}m{r}^2=4I$
Here is a link for the question number 4 which will help you to solve this question with proper explanation .
https://www.meritnation.com/ask-answer/question/explain-four-point-masses-each-of-value-m-are-placed-at-the/system-of-particles-and-rotational-motion/10718963

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