Question 292740
Let {{{w}}} = wind speed
Let {{{p}}} = speed of plane in still air
{{{p + w}}} = speed of plane flying with the wind
{{{p - w}}} = speed of plane flying against the wind
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Flying with the wind:
{{{d = (p + w)*4}}} km
(1) {{{d = (p + 20)*4}}} km
Flying against the wind:
{{{d = (p - w)*5}}} km
(2) {{{d = (p - 20)*5}}} km
Since {{{d}}} is the same in (1) and (2), they 
are equal to eachother
{{{(p + 20)*4 = (p - 20)*5}}}
{{{4p + 80 = 5p - 100}}}
{{{p = 180}}}
The plane's speed in still air is 180 km/hr