A two-digit number is such that the product of the digits is 8. When 18 is added to the number, then the digits are reversed. The number is:

• A. 42
• B. 81
• C. 24
• D. 18

Answer: (C)

Let the ten's and unit's digit be x and 8/x respectively. Then, (10x + 8/x) + 18 = 10 * 8/x + x 9x2 + 18x - 72 = 0 x2 + 2x - 8 = 0 (x + 4)(x - 2) = 0 x = 2 So, ten's digit = 2 and unit's digit = 4. Hence, required number = 24.