Correct Answer - Option 3 :

50 sec
**CALCULATION:**

1^{st} bike and train are travelling in opposite direction

⇒ Relative speed = 60 + 30 = 90 kmph = 25 m/sec

⇒ Length of train = 25 × 20 = 500 m

2^{nd} bike and train are travelling in same direction

⇒ Relative speed = 60 - 24 = 36 kmph = 10 m/sec

⇒ Time taken = 500/10 = 50 sec

We will consider the length of the bike as negligible, as it is very small as compared to the length of the train.