a, b - the numbers
[tex]\underline{\left\{\begin{array}{ccc}a+b=80\\a-b=42.5\end{array}\right}\ \text{add the both sides of the equation}\\.\ \ \ \ \ 2a=122.5\ \ \ \ |:2\\.\ \ \ \ \ \ a=61.25\\\\\text{substitute to first equation}\\\\61.25+b=80\ \ \ |-61.25\\b=18.75[/tex]
Answer: 61.25 and 18.75.
