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.

b. You find a vial that you know was collected prior to 1950, but you do not know the

exact year. Assume that the virus always has the same mutation rate. You sequence the

genome of this virus and find 10,000 single base pair differences between the sample

collected in 1950 an this sample that was collected at the unknown time. When do you

estimate this sample was collected and why?

Your help would be greatly appreciated! thank you