Question 755969
You can think of one train as standing still and the
other one approaching at the sum of their speeds.
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Let the speed of the slower train = {{{ s }}} mi/hr
The speed of the faster train = {{{ s + 21 }}} mi/hr
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{{{ 351 = ( s + s + 21 )*3 }}}
{{{ 2s + 21 = 117 }}}
{{{ 2s = 96 }}}
{{{ s = 48 }}}
{{{ s + 21 = 69 }}}
The speeds are 69 mi/hr and 48 mi/hr
check:
The faster train:
{{{ d[1] = 69*3 }}}
The slower train:
{{{ d[2] = 48*3 }}}
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{{{ d[1] = 207 }}} mi
and
{{{ d[2] = 144 }}} mi
{{{ 207 + 144 = 351 }}}
OK