The question needs better framing. It sounds as though you are assuming a constant distribution shape, since the viscosity average molecular weight and number average molecular weight are not the same moment of the distribution shape. Is this the question of a student ?
Your answer to this question is not appropriate. anyway, thanks for your attention. do you have any reference for this question??? be careful, viscosity and molecular weight ( see question again) # viscosity average molecular weight. Is this the answer of a Ph.D?