Let the unit of the primitive number be X and the decimal number be Y, then the primitive number = 10*y+x and x-y=3 according to the conditions.
The number of digits exchanged by ten digits = 10*x+y, then according to the condition (10 * y+x)+(10 * x+y) =121.
Combining these two equations, we can get x=7 y=4.
So the original number was 47.
English steps:
Suppose that the single digit is X and the ten digit is Y.
Then the original number is 10*y+x, and x-y=3.
The interchange number is10 * x+y.
So (10 * y+x)+(10 * x+y) =121
x=7 y=4
The original number was 47.