Question 950088
Let {{{ d }}} = the distance traveled 
by the slower plane until they are
{{{ 1890 }}} mi apart
Let {{{ t }}} = the time in hours for them 
to be {{{ 1890 }}} mi apart
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Equation for the slower plane:
(1) {{{ d = 540t }}}
Equation for faster plane:
(2) {{{ 1890 - d = 720t }}}
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Substitute (1) into (2)
(2) {{{ 1890 - 540t = 720t }}}
(2) {{{ 1260t = 1890 }}}
(2) {{{ t = 1.5 }}}
In 1.5 hrs they will be 1890 mi apart
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check:
(1) {{{ d = 540*1.5 }}}
(1) {{{ d = 810 }}}
and
(2) {{{ 1890 - d = 720t }}}
(2) {{{ 1890 - 810 = 720*1.5 }}}
(2) {{{ 1080 = 1080 }}}
OK