Question 1019370
Let r = wind speed.

In the outbound flight of the plane, its net speed will be 600 + r, while in the return trip it would be 600 - r.  (The plane would be flying parallel AGAINST the wind.)

In the outbound flight, using the formula d = r*t, the time duration of flight is {{{1500/(600+r)}}} hours.

In the return flight, after travelling {{{1500/(600+r)}}} hours, the plane has traveled {{{((600-r)*1500)/(600+r)}}} kilometers.  And there is still 300 kilometers to go.  Thus 

{{{1500 = 300 + ((600-r)*1500)/(600+r)}}}
==> {{{1200 = ((600-r)*1500)/(600+r)}}}
==> {{{4/5 = (600-r)/(600+r)}}}
Cross-multiplying, we get
2400+4r = 3000 - 5r
==> 9r = 600 ==> r = 66.67 kilometers per hour.