WAEC - Mathematics (2022 - No. 31)
The mean of a set of 10 numbers is 56. If the mean of the first nine numbers is 55, find the 10th number.
75
65
55
45
解释
10th number must be over 56 (as it increases the average)
55 x 9 = 495 = sum of first 9 numbers
56 x 10 = 560 = sum of all 10 numbers
560 - 495 = 65 = 10th number