Try this solution:
1. if 1st number is 'x', the 2d number is 'y', then it is possible to make up the system of two equation according to the condition:
[tex] \left \{ {{x-y=1} \atop {x+y=21}} \right. \ =\ \textgreater \ \ \left \{ {{x=11} \atop {y=10}} \right. [/tex]
2. one number is 11, another number is 10.
