Question 1120737
Let {{{r}}} = child’s rate on still sidewalk
Let {{{s}}} = rate of the moving sidewalk
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(1) {{{ r - s = 77/11 }}}
(2) {{{ r + s = 225/15 }}}
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(1) {{{ r - s = 7 }}}
(2) {{{ r + s = 15 }}}
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Add the equations
{{{ 2r = 22 }}}
{{{ r = 11 }}}
And
(1) {{{ 11 + s = 15 }}}
(1) {{{ s = 4 }}}
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Rate of child on still sidewalk
11 ft/sec
Rate of moving sidewalk
4 ft/sec