Given: A student finds the average of ten 2 digit numbers. By mistake, he writes one number with its digit interchanged. As a result that his average is 3.6 less than the correct answer. Concept used: Total weight = Average weight × Number of boxes Calculation: Let the digits of the number be ‘xy’ The number = 10x + y After interchanging the digit, the number becomes 10y + x The difference in average after interchanging the digits = 3.6 As there are 10 numbers, The difference between the numbers will be = 3.6 × 10 ⇒ 36 Difference in numbers = 9x – 9y ⇒ 9x – 9y ⇒ 36 Difference in digits, (x – y) = 4 ∴ The difference of the digits of the number in which he made the mistake is 4.