Question 898564
Let {{{ s }}} = the speed of the 1st plane
{{{ s + 150 }}} = the speed of the 2nd plane
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From 2 PM to 3 PM is {{{ 1 }}} hr
The 1st plane has a head start of:
{{{ d[1] = s*1 }}}
{{{ d[1] = s }}}
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Start a stop watch when the 2nd plane leaves
at 3 PM
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{{{ 4 }}} hrs after the 1st plane takes off will be
{{{ 4 - 1 = 3 }}} hrs on the stop watch
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Let {{{ d }}} = distance 2nd plane has traveled
when stopwatch reads {{{ 3 }}} hrs
{{{ d - s - 300 }}} = distance 1st plane 
has traveled in that time
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Equation for 1st plane:
(1) {{{ d - d[1] - 300 = s*3 }}}
(1) {{{ d - s - 300 = 3s }}}
(1) {{{ d = 4s + 300 }}}
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Equation for 2nd plane:
(2) {{{ d = ( s + 150 )*3 }}}
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(2) {{{ d = 3s + 450 }}}
Substitute (1) into (2)
(2) {{{ 4s + 300 = 3s + 450 }}}
(2) {{{ s = 150 }}}
and
{{{ s + 150 = 300 }}}
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The 2nd plane traveled:
(1) {{{ d = 4s + 300 }}}
(1) {{{ d = 4*150 + 300 }}}
(1) {{{ d = 600 + 300 }}}
(1) {{{ d = 900 }}} mi
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check answer:
(1) {{{ d - s - 300 = 3s }}}
(1) {{{ 900 - 150 - 300 = 3*150 }}}
(1) {{{ 450 = 450 }}}
and
(2) {{{ d = ( s + 150 )*3 }}}
(2) {{{ 900 = ( 150 + 150 )*3 }}}
(2) {{{ 900 = 300*3 }}}
(2) {{{ 900 = 900 }}}
OK