Question 561284
Let {{{ d }}} = distance into country in miles
Let {{{ t }}} = time to go into country in hrs
going in:
(1) {{{ d = 40t }}}
Coming back:
(2) {{{ d = 30*( 7 - t ) }}}
-----------------
(2) {{{ d = 210 - 30t }}}
Substitute (1) into (2)
(2) {{{ 40t = 210 - 30t }}}
(2) {{{ 70t = 210 }}}
(2) {{{ t = 3 }}}
and, since
(1) {{{ d = 40t }}}
(1) {{{ d = 40*3 }}}
(1) {{{ d = 120 }}} mi
The person can go 120 mi into country
check:
(2) {{{ d = 30*( 7 - t ) }}}
(2) {{{ 120 = 30*( 7 - 3 ) }}}
(2) {{{ 120 = 30*4 }}}
(2) {{{ 120 = 120 }}}
OK