Question 1155289
number of elements is 12.
the mean is the sum divided by the number of elements.
if the sum is the same, then the mean is the same.

sample 1 - everybody got 80.
sum is 12 * 80 = 960
mean is 960 / 12 = 80%


sample 2 - 6 got 70 and 6 got 90
sum is 6 * 70 + 6 * 90 = 410 + 540 = 960
mean is 960 / 12 = 80%


sample 3 - 3 got 60, 4 got 100, 1 got 40, 2 got 90, 1 got 75, 1 got 85
sum is 3 * 60 + 4 * 100 + 40 + 2 * 90 + 75 + 85 = 960
mean is 960 / 12 = 80%


as long as the sum of all 12 elements is 960, the mean will be 80.