Question 809333
Let {{{ s }}} = the speed of the plane in still air in mi/hr
Let {{{w }}} = the speed of the wind in mi/hr
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Flying into the wind:
(1) {{{ 1120 = ( s - w )*4 }}}
Flying with the wind:
(2) {{{ 1120 = ( s + w )*3.5 }}}
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(1) {{{ 4s - 4w = 1120 }}}
and 
(2) {{{ 3.5s + 3.5w = 1120 }}}
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Multiply (1) by {{{ 3.5 }}} and
(2) by {{{ 4 }}} and add the equations
(1) {{{ 14s - 14w = 3920 }}}
(2) {{{ 14s + 14w = 4480 }}}
{{{ 28s = 8400 }}}
{{{ s = 300 }}}
and, 
(1) {{{ 4s - 4w = 1120 }}}
(1) {{{ s - w = 280 }}}
(1) {{{ 300 - w = 280 }}}
(1) {{{ w = 20 }}}
The speed of the airplane in still air is 300 mi/hr
The speed of the wind is 20 mi/hr
check:
(2) {{{ 1120 = ( s + w )*3.5 }}}
(2) {{{ 1120 = ( 300 + 20 )*3.5 }}}
(2) {{{ 1120 = 320*3.5 }}}
(2) {{{ 1120 = 1120 }}}
You can check other equation