Ahelicopter flies 200 miles against the wind in 2 hours, with the wind, it can fly 300 miles in the same amount of time. find the speed of the helicopter in still air.

Against the wind, the helicopter's speed is (200 mi)/(2 h) = 100 mi/h.
With the wind, the helicopter's speed is (300 mi)/(2 h) = 150 mi/h.

The speed in still air is the average of these, (100 +150)/2 mi/h = 125 mi/h.