Let the digit in units place be x.
Then, the digit in tens place will be 2x.
So, the number will be
(10×2x)+(1×x)=20x+x=21x
When we interchange the digits, the number will be
(10×x)+(1×2x)=10x+2x=12x
It is given that,
21x+12x=66
⇒ 33x=66
⇒ x=6633
⇒ x=2
∴The digit in units place =2
The digit in tens place =4
The original number is 42.
Verification:
Original number =42
Interchanging the digits of 42, we get 24.
Sum of the original number and the interchanged number is 42+24=66.