Speed = distance / time.
Speed of him leaving the nest:
S = 100 / 20sec
5 m/s.
Catching the snake:
S2 = 50 / 5sec
10 m/s.
Average of 5& 10 = 7.5
7.5 m/s has to be the answer.
W = F x D
The work you put into a machine -- the input force -- is the force you apply times the distance you apply it. The work done by the machine equals the resisting weight times the distance it moves when you perform the work.
Average speed = Total distance/total time taken



