A person walks up a stalled escalator in $80 \mathrm{sec}$. When standing on the same escalator, now moving,…
A person walks up a stalled escalator in $80 \mathrm{sec}$. When standing on the same escalator, now moving, he is carried up in 20 s. The time taken by him to walk up the moving escalator is
$4 \mathrm{~s}$
$8 \mathrm{~s}$
$12 \mathrm{~s}$
$16 \mathrm{~s}$
Solution
Let distance travelled $=\mathrm{D}$
Speed of a person walks up $v_1=\frac{D}{80}$
Speed of escalator $v_2=\frac{D}{20}$ Total speed $\mathrm{v}=\mathrm{v}_1+\mathrm{v}_2 20$
$\begin{aligned} & =\frac{D}{80}+\frac{D}{20} \\ & =\frac{D+4 D}{80}=\frac{5 D}{80}\end{aligned}$
The time taken by him $\mathrm{t}_0$ walk up the moving escalator is
$\begin{aligned} & t=\frac{D}{5 D / 80} \\ & =16 \mathrm{~s}\end{aligned}$