You can estimate a tree's age in years by using the formula a = r w , where r is the tree's radius without bark and w is the average thickness of the tree's rings. the circumference of a white oak tree is 85 inches. the bark is 0.5 in. thick, and the average thickness of a ring is 0.2 in. estimate the tree's age and the area enclosed by the outer circumference of the widest ring. round answers to the nearest tenth.