Solution: The conventional approach

If the length of the bridge is x km and the train is y km away from the closer end of the bridge, in time y/60 hours, Robin runs x*40% km. So his speed is (x*40%) /(y/60).

In time (x + y)/60 hours, Roger runs x*60% km. So his speed is (x*60%)/ {(x + y)/60}.

As these speeds are the same, equating, we get y = 2x.

Now, as we have seen that the speed of Robin is (x*40%)/(y/60), it is (x*40%)/ (2x/60) = 12 km/ph.

 Maybe you like the Smart Scholars Method?

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