In this problem, 1975 is year zero. Therefore, 1995 is year 20 and 2005 is year 30.
Plug in these numbers for
f(years after 1975)=Initial value(1+(rate of increase))^(years/growth period)
Thus, the equation is
And you can solve from there
*note: f(30) is supposed to be function notation, not multiplication, just so you know. Oh, and in some equations, not this one, you need to make up for the first year by changing the equation to
to account for the extra growth