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Option 4 : 48 kmph
Given:
Initial speed = 60kmph
Speed after covering half distance = (60 – 20) kmph = 40 kmph
Formula used:
Average speed = Total distance travelled/total time taken
Average speed = 2xy/(x + y), where x = initial speed, y = final speed
Calculation:
Let distance between Aligarh and Lucknow be 120km
⇒ Time taken to travel half distance = 60/60 = 1 hrs
⇒ Time taken to travel remaining half distance = 60/40 = 1.5 hours
∴ Average speed = 120/(1 + 1.5) = 120/2.5 = 48 kmph
Short Trick:
Average speed = (2 × 60 × 40) / (60 + 40) = 4800/100 = 48 kmph
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