Question 835712
You can think of one of the trains as
standing still and the other train
moving at the sum of their speeds
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Let {{{ t }}} = the number of hours 
until they are {{{ 28.9 }}} miles apart
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{{{ 28.9 = ( 9 + 8 )*t }}}
{{{ t = 28.9 / 17 }}}
{{{ t = 1.7 }}}
In 1.7 hrs they will be 28.9 mi apart
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check answer:
Faster train:
{{{ d[1] = 9*1.7 }}}
{{{ d[1] = 15.3 }}} mi
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{{{ d[2] = 8*1.7 }}}
{{{ d[2] = 13.6 }}}
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{{{ d[1] + d[2] = 15.3 + 13.6 }}}
{{{ d[1] + d[2] = 28.9 }}} mi
OK