The question tests on proportionality and its application. Frame the equation and substitute the values. Then solve for the unknown. 

Solution:

Weight(W) = k * 4π·R^2 (k is an integer)
Given R1 = 0.15 ; w1 = 8 ;R2=0.3

=> w1/w2 = R1^2/R2^2

=> 8/w2 = (0.15)^2/ (0.3)^2

 => w2 = 32 gm