I am ashamed to admit that my mind cannot get it!!

Here is the problem. It requires working backward.

In the 1980s the rhinoceros population decreased by 485 from what it was in the 1970s. By the 1990s the population increased to 2 more than twice the population in the 1970s. By the 2000s the population dropped 25 rhinos to about 7 rhinos today. What was the population in 1970s?

Thanks in advance.