gmat prep question

[sarah99]

any takers?

[GMAT-HELP]

Let x be the increase every year.
Total height at the end of the 6th year = 4+6x
height at the end of the 4th year = 4+4x
Qn stem states that...4+6x = 6/5(4+4x)
=> x = 2/3
Ans D.