GMAT Prep - Test I - Q 31 Urgent; Please Help!

[puresillylily]

See the question attachment! Thank you so much!!
or below:
When a certain tree was first planted, it was 4 feet tall. and the height of the tree increases by a constant amount every 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


I have an exam on Saturday yet I cannot find anyone to help me with this question. Please kindly help me if you may!!!
Thank you so much!

Sincerely,
Puresillylily

[StanleyHuang]

假設每年長 t feet
4+6t = (4+4t)*(1+1/5)
第六年底的高度 = 第四年底的高度+ 1/5第四年底的高度
t = 2/3