The answer is 2. Imagine Maria can paddle 4 miles in either direction if there was no current in 2 hours. But if we add the current then we would subtract 2 when she goes upstream so she would get 4-2 which is 2 miles. And would get an extra 2 miles going downstream. So 4+2= 6 miles.
So the speed of the current is 2 mph.
Hope this helps!
