(b) A random mixture contains two materials A and B with mass fractions of 60% and 40%, respectively. Size distributions of particles A and B are given in table below. If the samples of 1 g are withdrawn from the mixture, what is the expected value of the standard deviation of the composition of the samples? Assume the particles are spherical and densities of A and B materials are 500 and 700 kg/m, respectively, Particle size, um 2000 1600 1400 1250 1000 750 500 XA 1 0.8 0.3 0.5 0.85 0.1 XB 0 1 0.65 0.45 0.2 0