Question 988659
The easiest way to solve this is to 
think of one car as standing still and
the other traveling at the sum of their speeds
( it doesn't matter which one is standing still )
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Let {{{ t }}} = time in hrs for them to meet
{{{ 642 =( 50 + 57 )*t }}}
{{{ 642 = 107t }}}
{{{ t = 6 }}}
It takes 6 hrs for them to meet
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check:
{{{ d[1] = 50t }}}
{{{ d[1] = 50*6 }}}
{{{ d[1] = 300 }}} mi
and
{{{ d[2] = 57t }}}
{{{ d[2] = 57*6 }}}
{{{ d[2] = 342 }}} mi
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{{{ d[1] + d[2] = 300 + 342 }}}
{{{ d[1] + d[2] = 642 }}} mi
OK