Question 737631
Let {{{ d }}} = Sarah's distance when they are 9 mi apart
{{{ d - 9 }}} = Mary's distance when they are 9 mi apart
Let {{{ t }}} = time in hrs passed after 8 AM when they are 9 mi apart 
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Sarah's equation:
(1) {{{ d = 10t }}}
Mary's equation:
(2) {{{ d - 9 = 4t }}}
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(2) {{{ d = 9 + 4t }}}
By substitution:
(2) {{{ 10t = 9 + 4t }}}
(2) {{{ 6t = 9 }}}
(2) {{{ t = 3/2 }}}'
8 AM + 3/2 hrs = 9:30 AM
check:
(1) {{{ d = 10*(3/2) }}}
(1) {{{ d = 15 }}}
and
(2) {{{ d - 9 = 4*(3/2) }}}
(2) {{{ d = 9 + 6 }}}
(2) {{{ d = 15 }}}
OK