We need to calculate a total TIME.
Let the time required to cover distance d at 20 mph be x. d is the ONE WAY distance, not the total distance traveled (which would be 2d).
Remember the formula d=rt: the distance traveled equals the rate times the time. Here, d = rx, and thus x = d/r = d/(20mph)
Let's begin by recognizing that d(going) = d(returning)
Then (20 mph)x = 6 + (16 mph)x
(20 mph)x - (16 mph)x = 6 miles
Then: (4 mph)x = 6 miles
or: x = 1.5 hours
It takes the bird 1.5 hours at 20 mph to cover distance d, which is 30 miles.
Thus, the total time spent flying is 1.5 hours (going) and [1.5 hours + (6 miles)/(16 mph) returning:
1.5 hours + 1.5 hours + 0.375 hours = 3.375 hours total.
The total distance covered is 2d, or 2(30 miles) = 60 miles.
It took me a while and a lot of experimentation to arrive at these results. Please, if my arguments here are not clear, ask questions.