I would follow the following steps to solve this problem:
1. Determine the amount of time it takes Bill to reach the group based on the difference in speed between Bill and the group (difference in speed is 3ft/s)
=> 120 ft / (6-3)ft/s = 40s
2. Determine the distance Bill travelled at a speed of 6ft/s and 3ft/s, respectively
=> 40s * 6ft/s = 240ft => Bill travelled 60ft at the lower speed of 3ft/s, which took him 60ft / 3ft/s = 20s
3. Determine Bill's average speed by dividing total distance by total time for the trip
=> 300 ft / (40s + 20s) = 5ft/s
Hence, the average total speed is 5ft/s.