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Problems on Numbers - General Aptitude

Q1:

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 18

B 24

C 42

D 81

ANS:B - 24

Let the ten's and unit digit be x and 8 respectively.
x
Then, 10x + 8 + 18 = 10 x 8 + x
x x
 10x2 + 8 + 18x = 80 + x2  9x2 + 18x - 72 = 0  x2 + 2x - 8 = 0  (x + 4)(x - 2) = 0  x = 2.