What is the speed of the plane with no wind ?

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)

Step-by-step explanation:

x+2y=10

6y=-3x+30

x =10 -2y

substitution

6y=-3x+30

6y=-3(10 -2y)+30

6y = -30 + 6y + 30

6y = 6y

everything disappears that means the lines are "the same line"

"co-incident" ... "infinite solutions"

Note" if you ended up with something like 3=0 then the lines are parallel