Question 860612
Let {{{ s }}} = the speed of the diesel train in mi/hr
How far does the diesel train get in 4 hrs?
{{{ d[1] = s*4 }}}
Let {{{ d }}}  = the distance the cattle car travels until 
the trains are {{{ 635 }}} mi apart
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If you start a stopwatch when the cattle train
leaves, it will show 3 hrs when the trains 
are {{{ 635 }}} mi apart
Diesel's equation:
(1) {{{ 635 - d - 4s = s*3 }}}
Cattle train's equation:
(2) {{{ d = ( s - 55 )*3 }}}
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Substitute (2) into (1)
(1) {{{ 635 - ( s - 55 )*3 - 4s = 3s }}}
(1) {{{ 635 -3s + 165 - 4s = 3s }}}
(1) {{{ 10s = 800 }}}
(1) {{{ s = 80 }}} mi/hr
and
{{{ 80 - 55 = 25 }}} mi9/hr
The diesel train's speed was 80 mi/hr
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check:
(1) {{{ 635 - d - 4s = s*3 }}}
(1) {{{ 635 - d - 4*80 = 80*3 }}} 
(1) {{{ 635 - d - 320 = 240 }}}
(1) {{{ d = 635 - 320 - 240 }}}
(1) {{{ d = 75 }}} mi
and
(2) {{{ d = ( s - 55 )*3 }}}
(2) {{{ d = ( 80 - 55 )*3 }}}
(2) {{{ d = 75 }}}
OK