Correct Answer - Option 4 : 42

**Concept used:**

Number = 10(Tens digit) + Unit digit

Ex:- 50 = 10 × 5 + 0 = 50

**Calculation:**

Let the unit and ten's digit of the number be x and 2x respectively

Number = 10(2x) + x = 21x

Interchanged number = 10x + 2x = 12x

Now, A/Q

21x - 12x = 18

⇒ 9x = 18

⇒ x = 2

Number = 21x = 21 × 2 = 42

**∴ The required number is 42**