Question 549899
The 1st train has a head start of 
{{{ 80*3 = 240 }}} mi
Start a stopwatch when the 2nd train leaves
Let {{{d}}} = distance 2nd train goes until they meet
Let {{{t}}} = time on stopwatch when they meet
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2nd train:
{{{ d = 100t }}}
1st train:
{{{ d - 240 = 80t }}}
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By substitution,
{{{ 100t - 240 = 80t }}}
{{{ 20t = 240 }}}
{{{ t = 12 }}}
This is 12 hrs after the 2nd train leaves at 6 PM + 3 = 9 PM
The 2nd train catches up with the 1st train at 9 AM the next day
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check answer:
{{{ d = 100*12 }}}
{{{ d = 1200 }}}
and
{{{ d - 240 = 80*12 }}}
{{{ d = 240 + 960 }}}
{{{ d = 1200 }}}
OK