Chapter 15.5, Problem 22E

As trees grow in diameter, the wood produced in spring and early summer is less dense and lighter in color than the wood produced in late summer and fall. The result is rings of light and dark wood known as annual tree rings. The thickness of the annual ring is determined largely by the weather that the tree experienced during that particular year. Hot, dry years tend to produce thin rings, whereas cool, wet years tend to produce thick rings. In a cross section of the 1001­year­old oak tree in Illustration 1, the annual rings for the 10 years from 1920 to 1929 were measured as follows:

Annual tree ring thickness (in mm)

1920	6
1921	8
1922	8
1923	8
1924	8
1925	7
1926	6
1927	6
1928	9
1929	11

What was the mean growth of the annual rings for the 1920s?

ILLUSTRATION 1