A certain tree was 100 meters tall at the end of 2017. It will grow 10% more than its previous height each year. How long has the tree grown from 2017 until the end of 2019? Think carefully first, then make a decision:
At the end of 2018, the tree will grow 100 x 10 / 100 = 10 meters more. Thus, its height at the end of 2018 is 100 + 10 = 110 meters. At the end of 2019, the tree will grow 110 x 10 / 100 = 11 meters more. So, its height at the end of 2019 is 110 + 11 = 121 meters. Therefore, the tree has grown 121 - 100 = 21 meters. So, the answer is 21.