Physics repost please clarify
posted by Mary .
The distance between earth and the moon can be determined from the time it takes for a laser beam to travel from earth to a reflector on the moon and back. If the roundtrip time can be measured to an accuracy of 0.19 of a nanosecond (1 ns = 109 s), what is the corresponding error in the earthmoon distance?

How far does light travel in .19ns?
distanceerror=c*deltatime
that will give you the distance error 
I think I am missing it. How do I get the change in time.

change in time is given. That time is infact the time error.