Question 899703
Think of one plane as not moving and the other 
one flying at the sum of their speeds
Let {{{ s }}} = the speed of the slower plane
{{{ 6000 = ( s + s + 200 )*5 }}}
{{{ 6000 = ( 2s + 200 )*5 }}}
{{{ 6000 = 10s + 1000 }}}
{{{ 10s = 5000 }}}
{{{ s = 500 }}}
{{{ s + 200 = 700 }}}
the speed of the slower plane is 500 mi/hr
check:
{{{ 6000 = ( 2s + 200 )*5 }}}
{{{ 6000 = ( 2*500 + 200 )*5 }}}
{{{ 6000 = 1200*5 }}}
{{{ 6000 = 6000 }}}
OK