Speed = V = Distance/ Time.
Let V be speed of the plane WITHOUT wind
Let v₁ be the SPEED OF THE WIND
When the wind is against, the resulting speed will be V-v₁ and if for the
wind (with), the resulting speed = V+v₁
V-v₁ = 1680/5 = 336 Mi/h
V+v₁ = 1680/4 = 420 Mi/h
Add up the right side and same with the left side
2V + 0 = 336 + 420
2V = 756
V = 378 Mi/h (with no wind)