When a certain tree was first planted, it was 4 feet tall, and the height of the tree increased by a constant amount each year for the next 6 years. At the end of the 6th year, the tree was 1/5 taller than it was at the end of the 4th year. By how many feet did the height of the tree increase each year? answer: 2/3 what does it meant when it says "the tree was 1/5 taller than it was at the end of the 4th "? 请教思路。谢谢。 | 
