The Doubling Tree Riddle

A tree doubled in height each year until it reached its maximum height over the course of ten years. How many years did it take for the tree to reach half its maximum height?

9 years.

Explanation:

The tree doubles in height each year.
In 10 years, it reaches full height.
This means that one year before reaching full height, it must have been half of its maximum height.
So, at year 9, the tree was half of its final height before doubling in its 10th year.

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