A train travelling with a speed of 60 kmph catches another train travelling in the same direction and then leaves it 120 m behind in 18 seconds. The speed of the second train is

Answer: Option C

Let speed of the 2^nd train is S m/sec.
And,
60 kmph $$=\frac{60\times 5}{18}=\frac{50}{3}$$    m/sec.
As trains are traveling in same distance, Then Relative distance,
$$\begin{array}{rl}& =\frac{60\times 5}{18}=\frac{50}{3}\\ & \frac{50}{3}-S=\frac{120}{18}\\ & \Rightarrow \frac{50-3S}{3}=\frac{20}{3}\\ & \Rightarrow 50-3S=20\\ & \Rightarrow 3S=50-20\\ & \Rightarrow 3S=30\\ & \therefore S=10\text{m/sec}\end{array}$$
Or, Speed of the 2^nd train = $$\text{10}\times \frac{18}{5}$$  = 36 kmph.