[tex]\it \dfrac{a}{b} = \dfrac{3}{7} \ \Rightarrow \ 7a = 3b \ \ \Rightarrow 7a - 3b = 0 \\ \\ 7a-3b = 0 \\ a + 3b = 120 \\ \\ ------^+ \\ 8a \ / = 120 \\ \\ a = \dfrac{120}{8} \\ \\ \boxed{a = 15} \\ \\ 15+3b=120 \\ 3b = 120-15 \\ 3b = 105 \\ \\ b = \dfrac{105}{3} \\ \\ \boxed{b = 35}[/tex]
