Question 886322
Think of this as 1 train standing still and
the other one traveling at the sum of their speeds
Let {{{ s }}} = the speed of the slower train
{{{ s + 15 }}} = the speed of the faster train
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{{{ 375 = ( s + s + 15 )*3 }}}
{{{ 375 = ( 2s + 15 )*3 }}}
{{{ 6s = 375 - 45 }}}
{{{ 6s = 330 }}}
{{{ s = 55 }}} 
{{{ s + 15 = 70 }}}
The trains are going 55 mi/hr and 70 mi/hr
check answer:
{{{ d[1] = 55*3 }}}
{{{ d[1] = 165 }}}
and
{{{ d[2] = 70*3 }}}
{{{ d[2] = 210 }}}
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{{{ d[1] + d[2] = 165 + 210 }}}
{{{ d[1] + d[2] = 375 }}} mi
OK