Question

At a metro station, a girl walks up a stationary escalator in $20 \,sec$. If she remains stationary on the escalator, then the escalator take her up in $30 \,sec$. The time taken by her to walk up on the moving escalator will be

• A. 25 sec
• B. 60 sec
• C. 12 sec
• D. 10 sec

Answer 1

Answer: (C)

12 sec

Answer 2

Let, the length of the escalator be $L$.
Velocity of the girl, $v_{g}=L / t_{1}=L / 20$
Velocity of the escalator, $v _{ e }= L / t _{2}= L / 30$
On moving escalator.
$v_{\text {net }}=v_{g}+v_{e}$
or, $v _{\text {net }}=( L / 20)+( L / 30)$
or, $v _{\text {net }}= L / 12$
Since, $v_{\text {net }}=L / t$
Therefore,
$L / t = L / 12$
$t =12 s$