Question 890089
Think of this as one of the planes flying away 
from the other at the sum of their speeds
and the other plane not moving
( parked on runway )
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Let {{{ s }}} = the speed of the faster plane in mi/hr
{{{ s - 50 }}} = the speed of the slower plane
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{{{ 275 = ( s + s - 50 )*( 1/2 ) }}}
{{{ 550 = 2s - 50 }}}
{{{ 2s = 600 }}}
{{{ s = 300 }}}
{{{ s - 50 = 250 }}}
The LA to SD plane's speed is 300 mi/hr
The LA to SF plane's speed is 250 mi/hr
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check:
The LA to SD plane:
(1) {{{ d[1] = 300*(1/2) }}}
The LA to SF plane:
(2) {{{ d[2] = 250*(1/2) }}}
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(1) {{{ d[1] = 150 }}} mi
(2) {{{ d[2] = 125 }}} mi
{{{ d[1] + d[2] = 150 + 125 }}}
{{{ d[1] + d[2] = 275 }}} mi
OK