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

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