Translation: A two-digit number, the unit number is 3 greater than the decimal number. If the unit number is exchanged with the decimal number, the obtained number is added to the original number, and the sum is equal to 12 1, thus finding the original number.

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.