How do I get the percentage average increase of registrations throughout the years?

year 1-year 2
% increase = (290-280)/280*100
year 2-year 3
% increase = (350-290)/290*100

X=year, f(X) is # of registrants for year X
g(X) = % increase for year X
g(X) = [f(X) - f(X-1)] / f(X-1)*100
for all X from 2002-2007 (there is no growth for 2001) find each g(X)
for example:
g(2002)= (290-280)/280*100% = 3.5%
g(2003)=(350-290)/290*100%= 20.7%
...
average % increase from 2002-2003 is (3.5+20.7)/2years = 12.1%