Question 889402
Let {{{ d }}} = the distance in km the faster plane travels
after they pass each other
{{{ 840 - d }}} km = the distance in km the slower plane
travels after they pass each other
Let {{{ s }}} = the average speed of the faster plane in km/hr
{{{ (3/4)*s }}} = the average speed of the slower plane 
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Slower plane's equation:
(1) {{{ 840 - d = (3/4)*s *(45/60) }}}
Faster plane's equation:
(2) {{{ d = s*(45/60) }}}
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Substitute (2) into (1)
(1) {{{ 840 - s*(45/60) = (3/4)*s *(45/60) }}}
(1) {{{ 840 - s*(3/4) = (3/4)*s *(3/4) }}}
(1) {{{ 840 - s*(3/4) = (9/16)*s }}}
Multiply both sides by {{{ 16 }}}
(1) {{{ 16*840 - 12s = 9s }}}
(1) {{{ 21s = 13440 }}}
(1) {{{ s = 640 }}}
and
{{{ (3/4)*640 = 480 }}}
The average speed of the faster plane is 640 km/hr
The average speed of the slower plane is 480 km/hr
check answer:
(1) {{{ 840 - d = (3/4)*s *(3/4) }}}
(1) {{{ 840 - d = (3/4)*640 *(3/4) }}}
(1) {{{ 840 - d = ( 9/16 )*640 }}}
(1) {{{ 840 - d = 360 }}}
(1) {{{ d = 480 }}} km
(2) {{{ d = s*(3/4) }}}
(2) {{{ d = 640*(3/4) }}}
(2) {{{ d = 480 }}} km
OK