(a) the number-average molecular weight. 22,500 * 0.04 + 37,500 * 0.07 + 52,500 * 0.16 + 67,500 * 0.26 + 82,500 * 0.24 + 97,500*0.12 + 112,500*0.08 + 127,500 * 0.03 = 73,800
(b) the weight-average molecular weight. 22,500*0.01 + 37,500*0.04 + 52,500 * 0.11 + 67,500 * 0.24 + 82,500 * 0.27 + 97,500 * 0.16 + 112,500 * 0.12 + 127,500 * 0.05 = 72,000
