Question from Gmatprep on sequence

[Flo20]

A certain tree was planted in a garden was 4 feet when planted. It grew by a constant amount each year. If the length of the tree at the 6th year was 1/5 more the length at the 4th year, at what rate was the tree growing each year?
A. 3/10
B. 2/5
C. 2/3
D.
E.

Hi there. The source of this question was Gmatprep but I don't remember the other options. I believe this is an arithmetic progression sequence and the way I solve it is as follows:
Beginning: 4
End of 1st year: 4+ r
End of 2nd year: 4+2r
End of 3rd year: 4+3r
End of 4th year: 4+4r
End of 5th year: 4+5r
End of 6th year: 4+6r

So, 6th year-4th year= 2r
2r=1/5. Therefore, r=1/10. Obviously, my slotion isn't among the options. Please what am I missing?

[Sage Pearce-Higgins]

Hi, you're confusing 1/5 of a foot with 1/5 of the height. Please search before you post, as this question is already addressed here: https://www.manhattanprep.com/gmat/foru ... ml#p105138