If you have learned the unknown, you can replace O and mouth with X and Y respectively. The original question can be converted into:

( 10x+y)- x =73

9x+y = 73；

Then write all integer solutions that meet the requirements, and x and y can only be 1 to 9;

When x=9, it does not meet the requirements.

When x=8, y= 1, which meets the requirements.

When x=7, y= 10, which does not meet the requirements.

So o stands for 8 and mouth stands for 1, which is the only solution.