Question 724724
Let {{{ t }}} = time in hrs spent going 60 mi/hr
Let  {{{ 8 - t }}} = time in hrs spent going 20 mi/hr
Let {{{ d }}} = distance traveled at 60 mi/hr
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(1) {{{ d = 60t }}}
(2) {{{ 240 - d = 20*( 8 - t ) }}}
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(2) {{{ 240 - d = 160 - 20t }}}
Multiply both sides by {{{ -1 }}}
(2) {{{ d - 240 = 20t - 160 }}}
Substitute (1) into (2)
(2) {{{ 60t - 240 = 20t - 160 }}}
(2) {{{ 40t = 80 }}}
(2) {{{ t = 2 }}}
and
{{{ d = 60*2 }}}
{{{ d = 120 }}}
{{{ 240 - 120 = 120 }}}
120 mi was traveled at an average of 20 mi/hr
check:
(2) {{{ 240 - d = 20*( 8 - t ) }}}
(2) {{{ 240 - 120 = 20*( 8 - 2 ) }}}
(2) {{{ 120 = 20*6 }}}
(2) {{{ 120 = 120 }}}
OK