It is known that the average and variance of the first 20 data are 60 and 20 respectively, and the average and variance of the last 20 data are 80 and 40 respectively
Total average: (60 * 20 + 80 * 20) / 40 = 70
Total variance s: s * 20-20 * 20-40 * 20 = 2 * 60 * 20-140 * 20 + 2 * 80 * 20-140 * 20
S=60