Given: Average of nine numbers = 60 Average of first five numbers = 55 and average of next three numbers = 65 Tenth number = Ninth number + 10 Concept used: Average = Total sum of all numbers / (Count of the numbers) Calculation: The sum of nine numbers = 60 × 9 = 540 The sum of the first five numbers = 55 × 5 = 275 The sum of the next three numbers = 65 × 3 = 195 ∴ Ninth number = (540 – 275 – 195) = (540 – 470) = 70 ∴ Tenth number = 70 + 10 = 80