A filmmaker wants to achieve an interesting visual effect by filming a scene through a converging lens with a focal length of 54.0 m. The lens is placed between the camera and a horse, which canters toward the camera at a constant speed of 7.8 m/s. The camera starts rolling when the horse is 41.0 m from the lens.

Question

Find the average speed of the image of the horse:
(a) during the first 2.0 s after the camera starts rolling and
(b) during the following 2.0 s.

Answer 1

1/f=1/di+1/do
or
dido=fdo+fdi
taking the implicit derivative ( I hope you are taking calculus)
do(di/dt+ di(do/dt)=f do/dt+f di/dt
you are given di/dt (-7.8), given f, and you want di/dt when do=41

di/dt(do-f)=do/dt(f-di) . At this point you need di when do=41, again, use the lens equation for that di.
Solve for di/dt

Answer 2

for average speed, it's just distance_traveled/time-taken