Question 650320
Let {{{ d }}} = distance to friend's house in miles
Let {{{ t }}} = time in hrs to drive to friend's house
given:
To friend's house:
(1) {{{ d = 40t }}}
Return trip:
(2) {{{ d = 45*( 4-t ) }}}
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Substitute (1) into (20
(2) {{{ 40t = 45*( 4-t ) }}}
(2) {{{ 40t = 180 - 45t ) }}}
(2) {{{ 85t = 180 }}}
(2) {{{ t = 36/17 }}}
(2) {{{ t = 2.1176 }}}
Plug this result back into (1) or (2)
(1) {{{ d = 40t }}}
(1) {{{ d = 40*( 36/17 ) }}}
(1) {{{ d = 84.706 }}} 
and {{{ 2d = 169.412 }}}
Mike traveled 169.412 mi
check:
(2) {{{ 84.706 = 45*( 4-t ) }}}
(2) {{{ 84.706 = 180 - 45t }}}
(2) {{{ 45t = 180 - 84.706 }}}
(2) {{{ 45t = 95.294 }}}
(2) {{{ t = 2.1176 }}}
OK