[tex]a+b=x \\\\ \boxed{a=10+b} \\\\ a;b \ \ \ \ direct \ proportional \ with \\ 5;3 \\\\ \frac{a}{5}=\frac{b}{3}=k \\ \\ \boxed{a=5k} \\ \boxed{b=3k} \\\\\\ a=10+b \\\\ 5k=10+3k \\\\ 5k-3k=10 \\\\ 2k=10 \\\\ \boxed{k=\frac{10}{2}=5} \\\\\\ a=5*5 \\\\ \boxed{\boxed{a=25}} \\\\ b=3*5 \\\\ \boxed{\boxed{b=15}} \\\\\\ a+b=x \\\\ 25+15=x \\\\ \boxed{\boxed{\boxed{x=40}}}}[/tex]
