[求助]PP-T1-PS-177

177.  18457-!-item-!-187;#058&011601

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?

(A)  3/10

(B)  2/5

(C)  1/2

(D)  2/3

(E)  6/5

想請問如果是問每年增長多少?