mean = sum / count, with count being the amount of papers corrected in this case. We want to find the sum of all the papers as well as the count to figure out the mean of all papers.
For Tony's papers,
mean = sum / count
50 = sum / 40
multiply both sides by 40 to isolate the sum
sum = 50 * 40 = 2000
For Alice's papers,
mean = sum / count
70 = sum / 20
multiply both sides by 20 to isolate the sum
70 * 20 = sum = 1400
The total sum of all 60 papers is equal to the sum of 40 papers + the sum of the remaining 20 papers, or 2000 + 1400 = 3400. The mean of the 60 papers is therefore
