A train overtakes two persons who are walking in the same direction in which the train is running, at the rate of 2 kmph and 4 kmph and passes them completely in 9 and 10 seconds respectively. The length of the train (in metres)

Speed of person 1 = 2 kmph
Relative speed of train with respect to person 1 = s - 2 kmph
Time taken by train to cross person 1 = 9 seconds = 9/3600 hours
Speed of person 2 = 4 kmph
Relative speed of train with respect to person 2 = s - 4 kmph
Time taken by train to cross person 2 = 10 seconds = 10/3600 hours
The distance covered is equal to the length of the train.
Since the length of train is constant, the product of speed and time n=must be the same.
$$(s-2) \times \frac{9}{3600} = (s-4) \times \frac{10}{3600}$$
$$(s-2)(9)=s-4(10)$$
$$9s - 18 = 10s -40$$
$$s - 22 kmph$$
Length of train = $$(s-2) \times \frac{9}{3600}$$
= $$20 \times \frac{9}{3600}$$
=$$\frac{1}{20} kms$$
= $$50 m$$
Hence Option D is the correct answer