We are working with the equation, d = rt (distance = rate x time)
Flying against the wind slows the plane down.
Flying with the wind allows the plane to fly faster.
Let's add one more variable, k, the speed of the wind.
The actual speed of the plane without wind is r.
The actual speed of the plane when travelling against the wind is r-k.
The actual speed of the plane when travelling with the wind is r+k.
We have two equations
2400 = (r-k)6
2400 = 4r
Use this 2nd equation to find r
r = 2400/4 = 600
Substitute this into the first equation.
2400 = (600-k)6
divide both sides by 6
400 = 600-k
We find that k = 200 km/hr.