Two trains $200$ m apart move toward each other at $20$ and $30$ m/s. They meet after

Two trains $200$ m apart move toward each other at $20$ and $30$ m/s. They meet after
  1. $4$ s
  2. $10$ s
  3. $50$ s
  4. $\dfrac{1}{4}$ s

Solution

Combined speed $= 50$ m/s. Time $= \dfrac{200}{50} = 4$ s.

Asked in: IMO

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