Question 1065193
I like to solve this type problem by saying
that one train is standing still and the other
one is travelling at the sum of their speeds
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Let {{{ s }}} = the speed of the faster train
{{{ s - 15 }}} = the speed of the slower train
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Add their speeds
{{{ 340 = ( s + s  - 15 )*4 }}}
{{{ 340 = ( 2s  - 15 )*4 }}}
{{{ 340 = 8s - 60 }}}
{{{ 8s = 400 }}}
{{{ s = 50 }}}
and
{{{ s - 15 = 35 }}}
Their speeds are 50 mi/hr and 35 mi/hr
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check:
{{{ d[1] = 50*4 }}}
{{{ d[1] = 200 }}}
and
{{{ d[2] = 35*4 }}}
{{{ d[2] = 140 }}}
and
{{{ d[1] + d[2] = 340 }}}
as it should be