Question 860246
Let {{{ L }}} = the length of the bridge in km
{{{ L/.1 }}} = the number of times the train would fit
into the bridge
{{{ L/.1 + 2 }}} = the number of times the entire train
fits in order to "pass through"
( note that I need the {{{ 2 }}} to account for both 
head and tail to get in and also to get out )
{{{ (( L/.1 + 2 )*.1 ) / 54 = 20 / 3600 }}}
{{{  L + .2 = 54*( 1/180 ) }}}
{{{ L = .3 - .2 }}}
{{{ L = .1 }}} km
The bridge is 100 m long
---------------
check:
The train travels 3 of it's lengths or .3 km
to pass through
{{{ .3 / 54 = .1/18 }}} hrs
{{{ ( 1/180 )*3600 = 20 }}} sec
OK
---------------------------
Hope I got it, and hope it's
not confusing