The average of 15 results is 21. The average of the first 7 of those is 21 and the average of the last 7 is 20. What is the 8th result?

25 31 28 22

28

Sum of 15 results = 15 x 21 = 315 Sum of 1st 7 results = 7 x 21 = 147 Sum of last 7 results = 7 x 20 = 140 8th result = 315 - 147 - 140 = 28