[tex]B \ bikes \to x\ (speed_1) \\A \ bikes \to x+15 \ (speed_2) \\\\ \boxed{S= \frac{D}{t}} \\\\ S\to speed \\ D \to distance \\ t\to time \\\\ The \ time \ is \ the \ same \ so \ the \ formula \ can \ be \ written \ like: \\\\ \boxed{\boxed{t=\frac{D}{S}}} \\\\ \frac{D_1}{S_1}=\frac{D_2}{S_2} \\\\ \frac{80km}{x+15}=\frac{50km}{x} \\\\ 80x=50(x+15) \\\\ 80x=50x+750 \\\\ 80x-50x=750 \\\\ 30x=750 \\\\ \boxed{x=\frac{750}{30}=25 \ kph}-speed \ of \ B \\\\ x+15=25+15 \\\\ \boxed{x+15=40 \ kph}-speed \ of \ A [/tex]
