(b) Let the digits at unit’s and ten's places be x and y, respectively Then, number = 10 y + x ∴ (10 y + x)(y + x) = 405 and (10x + y)(y + x) = 486 The number should be factors of 405, 405 =5x3x3x3x3 = 5 x 34 (15 x 27) and (45 x 9) are the two options only, 45 x 9 = (10 x 4 + 5) (4 + 5) Only satisfies the above two conditions. Hence, the required answer is 45.