Answer: 75%
Step-by-step explanation:
1960 to 1970 ... 100(.06-.04)/.04 = 50%
1970 to 1980 ... 100(.15-.06)/.06 = 150%
1980 to 1990 ... 100(.25-.15)/.15 = 67%
1990 to 2000 ... 100(.33-.25)/.25 = 32%
average = sum/4 = 75%
Always up, but 1970 to 1980 is so big an increase it dominates the statistics.
I hope this helps (I believe this is right)