Question 124087
They were 2/5 of the way down the bridge when they heard the train.  Assume that the train was coming from behind (otherwise this doesn't work).  Also assume that each jogger barely escapes.  The train is traveling 50 mph.

Let the length of the bridge be L.  Let the speed of the joggers be S.  

When the jogger that runs toward the short end of the bridge, and towards the train, reaches the end, he has travelled .4L.  The other jogger has, at the same time, reached a point at .8L. (He started at .4L, and runs the same speed as the other jogger).

The second jogger need to cover the remaining .2L before he is run over by the train, which has now started onto the bridge (since it just missed the first jogger).  The train is traveling at 50 mph.  The train will cover the length of the bridge in L/50 hours.  So,

{{{.2*L/S=L/50}}}

Divide both sides of the equation by L, and multiply both sides by 50:

{{{.2*L*50/(S*L)=L*50/(50*L)}}}

Simplify:

{{{.2*50/S=1}}}

Multiply both sides by S

{{{10*S/S=S}}}

or 

{{{10=S}}}

They were running at 10 mph.