First slide

Average Speed

Question

It takes one minute for a person standing on an escalator to reach the top from the ground. If the escalator is not moving, it takes him 3 minute to walk on the steps to reach the top. How long will it take for the person to reach the top if he walks up the escalator while it is moving?

Easy

A

2 minute
B

1.5 minute
C

0.75 minute
D

1.25 minute

Solution

Let d = distance between top and bottom
v_e - speed of escalator, v_m, = speed of man, then according to question
$\frac{d}{v_{e}} = 1 and \frac{d}{v_{m}} = 3$
Required time = $\frac{d}{v_{e} + v_{m}} = \frac{d}{d + (d / 3)} = \frac{3}{4} \min$

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