Answer: Option D, the actual speed of the wind is 16km/h.
Explanation:
Let's define two quantities:
Sp = Speed of the plane, that we assume it is constant.
Sw = Speed of the wind, that we assume it is constant.
When the plane flies with the wind, the total speed will be
Sp + Sw = 251km/h.
When the plane flies against the wind, the total speed will be:
Sp - Sw = 219km/h.
Then we have two equations:
Sp + Sw = 251km/h
Sp - Sw = 219km/h.
We want to find the speed of the wind, so we should isolate the speed of the plane in one of the equations, let's isolate it in the second equation:
Sp = 219km/h + Sw.
Now we can replace this into the other equation:
Sp + Sw = 251km/h.
(219km/h + Sw) + Sw = 251km/h.
2*Sw + 219km/h = 251km/h
2*Sw = 251km/h - 219km/h = 32km/h
Sw = (32km/h)/2 = 16km/h.
This is really close to the option B, but is not equal, so the correct answer will be D, none of the above.