RRB Group-D Model Paper 12 with answers for online practice

Concept:
Echo: If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo.
Echoes are heard due to the phenomenonofReflection of sound waves.
To hear the echo clearly, the reflecting object must be more than 17.2 m from the sound source for the echo to be heard by a person standing at the source.
Calculation:
Given,
Time taken to heard firstecho $\left({\mathit{t}}_1\right)$ = 2 sec
Time taken to heard second echo $\left({\mathit{t}}_2\right)$ = 3 sec
Velocity of sound (v) = 340 m/s
Let d be the distance between two cliffs, x is the distance between the man and cliff 2, and (d - x) is the distance between the man and cliff 1.
The speed of sound is

The distance between the man and cliff 2 is
$\Rightarrow 2\mathit{x}=\mathit{v}\times {\mathit{t}}_1$
$\Rightarrow \mathit{x}=\frac{\mathit{v}\times {\mathit{t}}_1}{2}$
$\Rightarrow \mathit{x}=\frac{340\times 2}{2}=340\mathit{m}$
The distance between the man and cliff 1 is
$\Rightarrow 2\left(\mathit{d}-\mathit{x}\right)=\mathit{v}\times {\mathit{t}}_2$
$\Rightarrow \left(\mathit{d}-\mathit{x}\right)=\frac{\mathit{v}\times {\mathit{t}}_2}{2}$
$\Rightarrow \left(\mathit{d}-340\right)=\frac{340\times 3}{2}=510$
⇒ d = (510 + 340) m = 850 m
Hence the correct option is 850 m.