If 30% molecules have M = 20,000, 40% molecules have M = 30, 000, rest of them have M = 60, 000. The PDI $=x\times {10}^{-1}$ Then  $x=$ 
 

$\begin{array}{l}{\overline{M}}_N=\frac{(30\times 20000)+(40\times 30000)+(30\times 60000)}{30+40+30}\\ =36000\\ {\overline{M}}_W=\frac{(30\times {20000}^2)+(40\times {30000}^2)+(30\times {60000}^2)}{(30\times 20000)+(40\times 30000)+(30\times 60000)}\\ =43333.33\end{array}$

Poly disparity index (PDI)   $=\frac{{\overline{M}}_W}{{\overline{M}}_N}$

$\begin{array}{l}=\frac{43333.333}{36000}\\ =1.20\\ =12\times {10}^{-1}\\ x=12\end{array}$