$${nt_1 = (n - 1)t_2\quad \Rightarrow\quad n\frac{s}{v_1} = (n - 1)\frac{s}{v_2} \quad\Rightarrow\quad \frac{n}{v_1} = \frac{n}{v_2} - \frac{1}{v_2} \quad\Rightarrow\quad}$$