Answer: The average growth rate of the urchin is 0.045 cm per year over the 80 year period between 30 and 110 years.
Explanation: In order to determine the average growth rate over the given time interval, we must first figure how how much the urchin grew over the specified period of time. The time period of growth is 80 years because
110 - 30 yields 80. Over that time, the urchin grew 3.6 cm because [tex]15.5 - 11.9 = 3.6[/tex].
We know that the urchin grew 3.6 cm over 80 years. Thus, in order to determine the average growth rate per year, we simply divide the amount the urchin grew by the time period. [tex]\frac{3.6}{80} = 0.045[/tex], meaning that the average growth rate of the urchin was 0.045 cm per year.