a, b - the numbers
[tex]\underline{+\left\{\begin{array}{ccc}a+b=23\\a-b=17\end{array}\right} |\text{add both sides of the equation}\\.\ \ \ \ \ 2a=40\ \ \ |:2\\.\ \ \ \ \ \ a=20\\\\\text{substitute to the first equation}\\\\20+b=23\ \ \ \ |-20\\b=3[/tex]
Answer: 20 and 3.
