Had to guess on this one since time was running and I was wasting my time solving this.
Some easy steps?
Little bit tricky...You need to read the problem carefully.
Ok, my take:
Let the rate of increment is r.
Original height is 4.
At the end of 1st year, height will be (4+r)
At the end of 4th year, height will be (4+4r)
At the end of 6th year, height will be (4+6r)
According to the problem, at the end of 6th year height of the tree will be 1/5 taller
than what it was at the end of 4th year....
Height at the end of 6th year = height at the end of 4the year + 1/5(height at the end of 4the year)
=> 4+6r = (4+4r) + 1/5(4+4r)
=> 2r = 4/5(1+r)
=> 10r = 4+4r
=> 6r = 4
=> r = 4/6
=> r = 2/3