I. Daska is situated at a distance of 15 miles from both Gujranwala and Sialkot. A bus travels from Gujranwala to Daska at an average speed of 25 miles per hour, from Daska to Sialkot at an average speed of 40 miles per hour and returns to Daska at an average speed of 30 miles per hour. Find the average speed for the entire trip.