GIVEN: When the digits of a 2-digit number are placed in the reverse order, the number decreases by 18. CONCEPT: Any number of the form ‘ab’ can be written as (10a + b). CALCULATION: Suppose the number is of the form: xy It can be written as: (10x + y) After reversing the number, it will be yx It can be written as: (10y + x) Now, according to the question: (10x + y) – (10y + x) = 18 ⇒ 9(x – y) = 18 ⇒ (x – y) = 2 Or x = (y + 2) We need to find all such numbers in form of ‘xy’ where x = y + 2 All possible numbers are: 20, 31, 42, 53, 64, 75, 86 and 97 Now, Average of all such numbers = [20 + 31 + 42 + 53 + 64 + 75 + 86 + 97] / 8 = 468 / 8 = 58.5