Question 201109
Let {{{s}}} = speed in still air
Let {{{j}}} = speed of jetstream
Flying with jetstream, plane's speed is:
{{{s + j}}}
Flying against jetstream, plane's speed is:
{{{s - j}}}
I can say:
{{{2800 = (s - j)*5}}} km
{{{2760 = (s + j)*3}}} km
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{{{2800 = 5s - 5j}}} 
{{{2760 = 3s + 3j}}}
Multiply both sides of the 1st equation by {{{3}}}
and both sides of the 2nd by {{{5}}}
{{{8400 = 15s - 15j}}} 
{{{13800 = 15s + 15j}}}
Add the equations
{{{22200 = 30s}}}
{{{s = 740}}} km/hr
and, since
{{{2760 = (s + j)*3}}}
{{{2760 = (740 + j)*3}}}
{{{2760 = 2220 + 3j}}}
{{{3j = 540}}}
{{{j = 180}}} km/hr