As The Wizard of Oz drifts away from Emerald City in his hot air balloon, Dorothy watches him leave. She notices that he is traveling at a constant altitude and at a speed of 20 miles per hour. She wonders how far he is above the ground. At one instant, the Scarecrow tells her that the angle of elevation to the Wizard's balloon is 24º. Three minutes later, the Scarecrow tells her that the angle of elevation to the balloon is now 21º. Using the information provided by the scarecrow, calculate the altitude of the balloon. Give your answer in feet, rounded to 3 decimal places.