Question 968028
The train "cross the bridge in 13 sec."
That means that 13 seconds after the  front of the train reached the bridge, the whole train had completely cleared the bridge.
During those {{{13 seconds=13/3600}}}{{{hours}}} ,
a train going at {{{72}}}{{{km/hr=72*1000}}}{{{m/hour}}} ,
would travel {{{72*1000m*(13/3600)}}}{{{hours=260m}}} .
That means that 13 seconds after the  front of the train reached the bridge,
the front of the train had traveled {{{260m}}},
and the back of the train, {{{150m}}} behind the front,
had just cleared the bridge.
{{{drawing(500,150,-200,300,-50,100,
line(-200,0,300,0),line(0,0,0,-50),
line(110,0,110,-50),green(rectangle(0,2,-68,17)),
green(rectangle(-150,2,-70,16)),green(rectangle(-70,2.5,-68,2.2)),
red(rectangle(260,2,192,17)),locate(-100,35,green(before)),
red(rectangle(110,2,190,16)),red(rectangle(190,2.5,192,2.2)),
locate(160,35,red(after)),arrow(0,-10,110,-10),
arrow(110,-10,0,-10),locate(20,-10,bridge),
arrow(110,-10,260,-10),arrow(260,-10,110,-10),
locate(140,-10,train=150m),arrow(110,40,0,40),
arrow(150,40,260,40),locate(115,45,260m)
)}}}
The length of the bridge is
{{{260m-150m=highlight(110m)}}} .