a. MRSCS = MUS/MUC = (2C1/3/3S1/3) / (S2/3/3C2/3) = (2C1/3/3S1/3) * (3C2/3/S2/3) = 2C/S = 2*1/2 = 1 (Putting C=1 and S=2)

b. MRSCS = MUS/MUC

or, MRSCS = {(1-a)Ca/Sa } / {aS(1-a)/C(1-a)}

or, 12.40 = (1-a)/a (putting C=S)

or, 12.40a = 1-a

or, 13.40a = 1

or, a = 1/13.40

or, a = 0.07