[問題]PERP 裡的PS,請教一下~

[balanana]

when a tree was first planted, it was 4 feet tall, and the hight 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 4th year.by how many feet did the hight of the tree increase each year?
1. 3/10
2. 2/5
3. 1/2
4. 2/3
5. 6/5

我算的不是答案4耶~
可否請教答案2/3是怎麼算出來的阿??
謝謝~

[jinoliway]

4+4a=x
4+6a=6x/5
解聯立 a=x/10 帶回任一式
得a=2/3
報告完畢

[balanana]

謝謝你~看來我粗心 了..