Question

A train moves past a telegraph post and a bridge 120 m long in 8 seconds and 20 seconds respectively. What is the speed of the train​

Answer 1

length = 8s

20 = 8s + 120/s

20s-8s = 120

s = 10

Answer 2

Given,

The length of the bridge$\[ = 120\]$

The time taken by the train to cross the telegraph post$\[ = 8\]$

The time taken by the train to cross a bridge$\[ = 20\]$  

Solution,

Assume that the length of the train is L and the speed of the train is v m/s.

Apply condition first.

$\[\begin{array}{l} \Rightarrow \frac{L}{v} = 8\\ \Rightarrow L = 8v\end{array}\]$

Apply condition second.

$\[ \Rightarrow \frac{{L + 120}}{v} = 20\]$

Now apply the value of$\[L = 8v\]$ in the above equation.

Therefore,

$\[\begin{array}{l} \Rightarrow \frac{{8v + 120}}{v} = 20\\ \Rightarrow 8v + 120 = 20v\\ \Rightarrow 12v = 120\\ \Rightarrow v = 10\end{array}\]$

Hence, the speed of the train is $\[10\]$ m/s.