GMAT Math

vishal_2804 wrote:When a certain tree was first planted, it was 4 feet tall, and height of he tree increased by a constant amount each year for the next 6 yrs. At the end of 6th year, the tree was (1/5) taller than it was at the end of the 4th yr. By how many feet did the height of the tree increase each year?

Let us assume, the height by which the tree increased constantly each year be x ft.
At the end of the 1st year, height of tree = 4 + x
At the end of the 2nd year, height of tree = 4 + 2x
...
At the end of the 6th year, height of the tree = 4 + 6x
It is given that at the end of the 6th year, the tree was 1/5 taller than it was at the end of the 4th year.
So, (4 + 6x) = (4 + 4x) + 1/5 of (4 + 4x)
--> 6x = 4x + (4 + 4x)/5
--> 2x = (4 + 4x)/5
--> 10x = 4 + 4x
--> 6x = 4
--> x = 2/3

The correct answer is D.