Question 581312
Let {{{ s }}} = Sarah's speed in mi/hr
{{{ s + 7 }}} = Dave's speed in mi/hr
given:
Sarah gets a head start of {{{ d = 2s }}}
They are {{{ 50 - 2s }}} miles apart when Dave leaves
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Now I can think of Sarah as standing still and 
Dave moving towards her at the sum of their speeds
If Sarah is on her bike for {{{ 4 }}} hrs, then
when Dave starts, they both travel for {{{ 2 }}} more hrs
{{{ 50 - 2s = ( s + s + 7 )*2 }}}
{{{ 50 - 2s = ( 2s + 7 )*2 }}}
{{{ 50 - 2s = 4s + 14 }}}
{{{ 6s = 36 }}}
{{{ s = 6 }}}
{{{ s + 7 = 13 }}}
Sarah's speed is 6 mi/hr
Dave's speed is 13 mi/hr
check:
Their separate distances are:
Dave:
{{{ ( s + 7)*2 = 13*2
{{{ 13*2 = 26 }}} mi
Sarah:
{{{ 2s + s*2 = 4s }}}
{{{ 4s = 4*6 }}}
{{{ 4*6 = 24 }}} mi
and
{{{ 24 + 26 = 50 }}} mi
OK