Question 1128724
Let {{{ r }}} = child rate of running
Let {{{ s }}} = rate of the sidewalk
{{{ r + s = 120/8 }}} ft/sec
{{{ r - s = 56/8 }}} ft/sec
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{{{ r + s = 15 }}}
{{{ r - s = 7 }}}
----------------- Add
{{{ 2r = 22 }}}
{{{ r = 11  }}}
and
{{{ 11 + s = 15 }}}
{{{ s = 4 }}}
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Child's rate is 11 ft/sec
Moving sidewalk's rate is 4 ft/sec