Question 1070748
You can think of this as one of the trains standing
still and the other one traveling at the sum of their speeds
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Let {{{ t }}} = time in hrs for the trains to be 354 km apart
{{{ 345 = ( 55 + 60 )*t }}}
{{{ 345 = 115t }}}
{{{ t = 3 }}} 
They will be 345 km apart in 3 hrs
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( I used {{{ 345 }}} because it makes the time come out a whole number )
check answer:
{{{ d[1] = 55t }}}
{{{ d[1] = 55*3 }}}
{{{ d[1] = 165 }}} km
and
{{{ d[2] = 60t }}}
{{{ d[2] = 60*3 }}}
{{{ d[2] = 180 }}} km
and
{{{ d[1] + d[2] = 165 + 180 }}}
{{{ d[1] + d[2] = 345 }}}
OK