Question 956012
You can think of this as one girl standing still
and the other one traveling at the sum 
of their speeds
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Let {{{ s }}} = Dory's speed in km/hr
{{{ 20 = ( s + s + 6 )*.5 }}}
{{{ 20 = s + 3 }}{}
{{{ s = 17 }}}
{{{ s + 6 = 23 }}}
Dory's speed is 17 km/hr
Mida's speed is 23 km/hr
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check:
{{{ d[1] = 17*.5 }}}
{{{ d[1] = 8.5 }}} km
and
{{{ d[2] = 23*.5 }}}
{{{ d[2] = 11.5 }}}
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{{{ d[1] + d[2] = 8.5 + 11.5 }}}
{{{ d[1] + d[2] = 20 }}} km
OK