The average molecular weight of the sample will be (b) 4600

Explanation:

The average molecular weight is given by the relation,

W = (WM + WM)/(W + W)

W = 2

W = 3

M = molar mass of 1st fraction = 100 x 28 = 2800

M = molar mass of 1st fraction = 200 x 28 = 5600

putting these values in the given eqn

W = (2x2800 + 3x5600)/(2+3)

= (5600 + 16800)/5

= 22400/5

= 4480

≈ 4600

Hence the average molecular weight of the sample will be 4600