We can plug in the answer choices, which represent the increase in feet each year.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
Let h = height of tree.
Answer choice C: increase = 1/2 each year.
After 4 years, h = 4 + 4(1/2) = 6.
After 6 years, h = 6 + 2(1/2) = 7.
6 years - 4 years = 7-6 = 1.
1/5 * (4 years) = 1/5 * 6 = 6/5. Doesn't work.
1< 6/5, so the yearly increase must be greater.
Answer choice D: increase = 2/3 each year.
After 4 years, h = 4 + 4(2/3) = 20/3.
After 6 years, h = 20/3 + 2(2/3) = 24/3.
6 years - 4 years = 24/3 - 20/3 = 4/3.
1/5 * (4 years) = 1/5 * 20/3 = 4/3. Success!
The correct answer is D.
do not use each in SC part of GMAT for such particular case, as each is used as one out of the two, and every may be used for more than two terms 
