It takes 50 seconds.
We will use the formula d = rt for this, where d is the distance traveled, r is the speed and t is the amount of time.
For land travel,
d = 50t
Since it takes 200 seconds by land, this gives us
d = 50(200) = 10,000
Now we use this same distance in the formula for air travel:
d = 200t
10,000 = 200t
Divide both sides by 200:
10,000/200 = 200t/200
50 = t