Let the usual speed of bus = $$7x$$ km/hr Total distance = 200 km Distance travelled at usual speed = $$frac{2}{5} imes200=80$$ km and distance travelled at slow speed = $$200-80=120$$ km Slow speed = $$frac{6}{7} imes7x=6x$$ km/hr According to ques, => $$frac{120}{6x}-frac{80}{7x}=frac{50}{60}$$ => $$frac{20}{x}-frac{80}{7x}=frac{5}{6}$$ => $$frac{60}{7x}=frac{5}{6}$$ => $$x=frac{60}{7} imesfrac{6}{5}=frac{72}{7}$$ $$ herefore$$ Usual speed of bus = $$7 imesfrac{72}{7}=72$$ km/hr => Ans - (B)