These type of questions appear fairly regularly on the forum..... they are a little tricky....you must remmeber to take into acount TIME not just distance and speed.
Rate x time = distance
distance /rate = time
((150 laps x 3 mi/lap )/180m/hr + (50 laps x 3 mi/lap)/200mi/hr ) = 2.5 hr + .75 hr = 3.25 hr for 600 miles
distance/time = rate 600 m / 3.25 hr = 60000/325 = 2400/13 = 184.62 m/hr