Help me understand this genetics problem please?

You are in charge of estimating the mutation rate in an emerging human virus. You are

given a very old sample collected in 1950 that contains DNA from that virus as well as a

DNA sample of the same virus collected fifty year later, in 2000. You have the complete

genome sequences for both samples. The genome size of this virus is 20,000 base pairs.

There are 10,000 replication events per year. You find 2,000 single base pair differences

between the samples collected in 1950 and 2000. Assume the entire genome is evolving

under neutral expectations.

a. What is mutation rate on a per-replication and per year basis in the virus (include

units)?

Your help would greatly be appreciated, thank you!! (This is NOT my homework - it is an exam review)