Question 1109107
Johnny's headstart in miles is:
{{{ d[1] = 70*1 }}}
{{{ d[1] = 70 }}} mi
Start a stopwatch when Jackie leaves
Let {{{ t }}} = the time in hrs on the stopwatch
when Jackie catches up with Johnny
Let {{{ d }}} = the distance Jackie travels
until he catches up with Johnny
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Johnny's equation:
(1) {{{ d - d[1] = 70t }}}
Jackie's equation:
(2) {{{ d = 80t }}}
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(1) {{{ d - 70 = 70t }}}
Plug (2) into (1)
(1) {{{ 80t - 70 = 70t }}}
(1) {{{ 10t = 70 }}}
(1) {{{ t = 7 }}} hrs
and
(2) {{{ d = 80*7 }}}
(2) {{{ d = 560 }}} 
Both were 560 mi from the starting point
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check:
(1) {{{ d - 70 = 70t }}}
(1) {{{ d = 70*7 + 70 }}}
(1) {{{ d = 490 + 70 }}}
(1) {{{ d = 560 }}}
OK