The average of 11 observations is 10, that of the first 6 being 9 and that of the last 6 being 11. What is the 6th observation?

11 8 9 10

10

Sum of 11 observations = 11 × 10 = 110 Sum of 1st 6 observation = 6 × 9 = 54 Sum of last 6 observation = 6 × 11 = 66 Total sum = 54 + 66 = 120 11th observation = 120 - 110 = 10