Let the digit at the unit's place be x.
Digit at tens place = 2x
Original number = (2x)×10 + x = 21x
Number obtained by interchanging the digits = (x)×10 + 2x = 12x
From the given information,
∴ The unit's digit = 2.
Tens Digit = 2 × 2 = 4.
Hence, the number is 42.