When Mrs. Johnson's class planted a tree, it was 9 feet high. Each year, including the fourth year, the height of the tree increased by x feet. During the fourth year, the height of the tree increased by one-sixth the height it had reached at the end of the third year. How many feet high was the tree at the end of the fourth year?
(Source: Adapted from Mathematics Teaching in the Middle School, Mar-Apr 1997)
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