Question

At a metro station, a girl walks up a stationary escalator in $20sec$. If she remains stationary on the escalator, then the escalator take her up in $30sec$. 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)

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,
$L/t=L/12$
$t=12s$