Question 640634
Mary has a head start of {{{ 60*1 = 60 }}} mi
Start a stopwatch when Jim leaves
Let the time on the watch = {{{ t }}} hrs when
Jim catches Mary
Let {{{ d }}} = distance that Jim travels 
to catch Mary
Mary's equation:
(1) {{{ d - 60 = 60t }}}
Jim's equation:
(2) {{{ d = 70t }}}
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Substitute (2) into (1)
(1) {{{ 70t - 60 = 60t }}}
(1) {{{ 10t = 60 }}}
(1) {{{ t = 6 }}}
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It will take 6 hrs for Jim to catch Mary
check:
(2) {{{ d = 70t }}}
(2) {{{ d = 70*6 }}}
(2) {{{ d = 420 }}} mi
and
(1) {{{ d - 60 = 60*6 }}}
(1) {{{ d = 360 + 60 }}}
(1) {{{ d = 420 }}} mi
OK