Question 167243
Let speed of jetstream = {{{s}}} km/hr
Let the speed of the plane in still air = {{{p}}} km/hr 
{{{d[1] = r[1]*t[1]}}}
(1) {{{5250 = (p - s)*7}}}
{{{d[2] = r[2]*t[2]}}}
(2) {{{6900 = (p + s)*6}}}
There are 2 equations and 2 unknowns, so I  should
be able to solve
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(1) {{{5250 = (p - s)*7}}}
(1) {{{5250 = 7p - 7s}}}
(2) {{{6900 = (p + s)*6}}}
(2) {{{6900 = 6p + 6s}}}
Divide both sides of (1) by {{{7}}} and
divide both sides of (2) by {{{6}}}
(1) {{{750 = p - s}}}
(2) {{{1150 = p + s}}}
Add the equations
{{{1900 = 2p}}}
{{{p = 950}}}
And, substituting back into (1)
(1) {{{750 = p - s}}}
(1) {{{750 = 950 - s}}}
{{{s = 200}}}
The speed of the jet in still air is 950 km/hr
The speed of the jetstream is 200 km/hr
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check answer:
(1) {{{5250 = (p - s)*7}}}
(1) {{{5250 = (950 - 200)*7}}}
{{{5250 = 750*7}}}
{{{5250 = 5250}}}
(2) {{{6900 = (p + s)*6}}}
(2) {{{6900 = (950 + 200)*6}}}
{{{6900 = 6900}}}
OK