Question 918367
David's head start is:
{{{ d[1] = 20*( 3/4 ) }}} ( 45 min = 3/4 hr )
{{{ d[1] = 15 }}} km
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Start a stopwatch when Kristi leaves
Let {{{ t }}} = time on the stopwatch in hrs
when she catches David
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Let {{{ d }}} = distance  in km from Boston when 
she catches David
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David's equation:
(1) {{{ d - 15 = 20t }}}
Kristi's equation:
(2) {{{ d = 25t }}}
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Substitute (2) into (1)
(1) {{{ 25t - 15 = 20t }}}
(1) {{{ 5t = 15 }}}
(1) {{{ t = 3 }}} hrs
and
(2) {{{ d = 25t }}}
(2) {{{ d = 25*3 }}}
(2) {{{ d = 75 }}} km
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Kristi catches him 75 km from Boston
check:
(1) {{{ d - 15 = 20t }}}
(1) {{{ 75 - 15 = 20*3 }}}
(1) {{{ 60 = 60 }}}
OK