a) 15 hours
b) 24 hours
c) 25 hours
d) 30 hours

Answer: d
Explanation: Let Usual Speed = x kmph
Speed of bus after increasing the speed = (x + 14)kmph
A ____________1680km ____________B
In first case, Time taken to covered the distance 1680 km = $$\frac{{1680}}{x}$$ . . . . . (1)
In Second Case, Time Taken to covered the distance 1680 km = $$\frac{{1680}}{{x + 14}}$$ . . . . . (2)
Time difference = 6 Hours.
So,
$$\eqalign{ & {\frac{{1680}}{x} – \frac{{1680}}{{ {x + 14} }}} = 6 \cr & 1680 {\frac{{14}}{{ {{x^2} + 14x} }}} = 6 \cr & 280 \times 14 = {x^2} + 14x \cr & {x^2} + 70x – 56x – 3920 = 0 \cr & x\left( {x + 70} \right) – 56\left( {x + 70} \right) = 0 \cr & x = – 70,56. \cr & {\text{hence,}}\,{\text{speed}}\,{\text{of}}\,{\text{minibus}}\,{\text{is}}\,56\,km/h \cr & {\text{put}}\,x = 56\,{\text{in}}\,{\text{equation}}\,(2) \cr & T = \frac{{1680}}{{56}} \cr & \,\,\,\,\,\, = 30\,{\text{hours}} \cr} $$