Question 1094814
From the time the front enters until the 
rear of the train exits, the train has traveled
{{{ 400 + x }}} miles in {{{ 30 }}} sec
Rate of train = {{{ ( 400 + x ) / 30 }}}
Rate = {{{ 40/3 + x/30 ) }}}
The stationary light tells you that the rate 
of the train is {{{ x/10 }}}
I can say now:
{{{ 40/3 + x/30 = x/10 }}}
{{{ 400 + x = 3x }}}
{{{ x = 200 }}}
The train is 200 m long
I get (d) for answer
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