Question 1074360
Let {{{ s }}} = his walking speed
{{{ 2s }}} = his running speed
Let {{{ d }}} = his distance walking
{{{ 21 - d }}} = his distance running
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Equation for walking:
(1) {{{ d = s*2 }}}
Equation for running:
(2) {{{ 21 - d = 2s*2 }}}
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(1)  {{{ d = 2s }}}
(2) {{{ 21 - d = 4s }}}
Substitute (1) into (2)
(2) {{{ 21 - 2s = 4s }}}
(2) {{{ 6s = 21 }}}
(2) {{{ s = 3.5 }}}
and
{{{ 2s = 7 }}}
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He runs 7 mi/hr
He walks 3.5 mi/hr
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check:
(1)  {{{ d = 2s }}}
(1)  {{{ d = 7 }}}
and
(2) {{{ 21 - d = 4s }}}
(2) {{{ 21 - d = 14 }}}
(2) {{{ d = 7 }}}
OK