Question 788093
Let {{{ d }}} = your friends distance in miles when 
you catch up
Let {{{ t }}} = time in hrs when you catch up
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Your equation:
(1) {{{ d + 3 = 18t }}} 
Friend's equation:
(2) {{{ d = 12t }}}
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Substitute (2) into (1)
(1) {{{ 12t + 3 = 18t }}}
(1) {{{ 6t = 3 }}}
(1) {{{ t = 1/2 }}}
You catch up in 1/2 hour
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check:
(1) {{{ d + 3 = 18*(1/2) }}}
(1) {{{ d = 9 - 3 }}}
(1) {{{ d = 6 }}}
and
(2) {{{ d = 12*(1/2) }}}
(2) {{{ d = 6 }}}
OK