A two - digit number is such that the product of its digits is 35. If 18 is added to the number, the digits interchange their places. Find the number.
Let the two - digit number be xy (i.e. 10x + y).
After reversing the digits of the number xy, the new number becomes yx (i.e. 10y + x).
According to question -
xy = 35
⇒ x = 35/y.....(1)
and,
(10x + y) + 18 = (10y + x)
⇒ 9x - 9y = - 18
⇒ x - y = - 2.....(2)
Substituting the value of x in equation (2), we get -
⇒ 35 - y2 = - 2y
⇒ y2 - 2y - 35 = 0
⇒ y2 - 7y + 5y - 35 = 0
⇒ y(y - 7) + 5(y - 7) = 0
⇒ (y + 5)(y - 7) = 0
∴ y = 7
[y = - 5 is invalid because digits of a number cannot be negative.]
Substituting the value of y in equation (1), we get -
x = 5
Thus, the required number is 57.