A car travels from city A to city B with a constant speed. If the speed of the car is increased by 20 km/h it will reach destination 1 hour earlier. On the other hand, if its speed decreases by 20 km/h, it reaches 2 hours later than scheduled time. What is the usual time if it travels with routine speed?
A2 hr
B4 hr ✓ Correct
C8 hr
D10 hr
Correct answer: (B) 4 hr
Explanation
The usual time is 4 hours.
Let usual speed be v and time t, with distance d = v t.
Faster : (v + 20)(t - 1) = v t gives 20t - v - 20 = 0, so v = 20t - 20.
Slower : (v - 20)(t + 2) = v t gives 2v - 20t - 40 = 0, so v = 10t + 20.
Equating : 20t - 20 = 10t + 20, so 10t = 40.
Hence t = 4 hours.
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