A plan left 40 minutes late due to bad whether and order to reach its destination 1600 km away in time, it had increase its speed by 400 kmph from its usual speed. Find the usual speed of the plane?

Let usual speed be X kmph, then new speed will be (x + 400) kmph.
Time taken to cover 1600 km with speed X kmph,
= $$\frac{{1600}}{{\text{x}}}$$
Time taken to cover 1600 km with Speed (x + 400) kmph,
= $$\frac{{1600}}{{{\text{x}} + 400}}$$
Now,
Time difference = 40 minutes.
$$\frac{{1600}}{{\text{x}}} - \frac{{1600}}{{{\text{x}} + 400}} = \frac{{40}}{{60}}$$     hours
x² + 400x - 960000 = 0 On solving,
x = -1200, 800
Speed cannot be negative, So usual speed will be 800 km/hour