We know that Bill covers 120 feet at 3 feet per sec.
Thus, the time taken to cover the walkway will be 1203→40 seconds.
Since his default walking speed is 6, then the amount he can cover in 40 seconds is 6⋅40→240.
Now he has 60 feet left to cover, which will take him 603→20 seconds.
Since the Average speed=The Total distance/TheTotal time, we get 30020+40→30060=5 feet per second is his average rate. 