MensaNumber wrote:
A train traveled from station A to station B at an average speed of 80 kmph and then from Station B to Station C at an average speed of 60 kmph. What was the average speed from A to C?
1. The distance from A to C four times the distance from B to C
2. The amount of time it took to travel from A to B is four time that of time it took to travel from B to C
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Average speed = total distance/ total time.
1) A-----B-------C
let bc=x, therefore AC=4x, and AB=3x
thus average speed = 4x/{(3x/80)+(x/60)}
we can easily calculate the value of avg. speed from the above exp. hence sufficient
2) A---------B----------c
let distance between AB=x and BC=y
x/80=4(y/60)
x=(16/3)y-------------1)
average speed = x+y/{(x/80)+(y/60)}
we can substitute the value of x in terms of y in the above expression to find out the average speed. hence sufficient
therefore correct answer should be D