a) 600 km/hour
b) 400 km/hour
c) 1000 km/hour
d) 800 km/hour
Answer: d
Explanation: 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,
= $$\frac{{1600}}{{{\text{x}} + 400}}$$
Time difference = 40 minutes.
$$\frac{{1600}}{{\text{x}}} – \frac{{1600}}{{{\text{x}} + 400}} = \frac{{40}}{{60}}$$ hours
x2 + 400x – 960000 = 0
x = -1200, 800
Speed cannot be negative, So usual speed will be 800 km/hour
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