15mph
If the man turns and runs toward point A, he will cover
3/8 of the length of the bridge in the time that it takes
the train to reach A.
If the man runs forward toward point B, what part of the bridge
will he cover before the train reaches A? Well, he will cover
3/8 of the bridge, only heading forward toward B. This will put
him 3/8 + 3/8 = 6/8 = 3/4 of the way across the bridge by the
time the train reaches A.
since we know that the man and the train will meet at B, this
means that in the time it takes the man to run the remaining
1/4 of the bridge, the train will cover the entire length of
the bridge.
If it takes the man the same time to cover 1/4 of the bridge
that it takes the train to cover the whole bridge, then the train
must be going four times as fast as the man. Another way of saying
this is that the man runs at 1/4 the speed of the train.
Since the train's speed is known to be 60 mph, this means that
the man runs at (1/4) 60 = 15 mph.
