It traveled 200 m in 50 seconds. 200/50 can be simplified to 4 m/s!
The velocity is -4 m/s (negative because it travelled from 100 to -100 or backwards)
B. they both involve wave interaction.
The focal length of the eye-lens system will be exactly 2.0 cm. The lens thickness is altered depending on the distance of the object being viewed so that the focal length is ALWAYS equal to the lens-retina distance. If this is not true fr an individual, they need glasses :)
The motion of the light is a uniform motion with constant speed

, therefore we can use the basic relationship between speed, space and time:

(1)
where S is the distance covered and t is the time taken. The light takes t=1.3 s to travel from the moon to Earth, therefore by rearranging eq.(1) we can find the distance between the Moon and the Earth: