The easiest way to do this problem is to recognize that there is a constant rate of work. Determine the rate of work and use that value to answer the question.

r  =  x / t
r  =  (3 trees) / (25 min)  =  [(3/25) tree/min] (60 min/hr)  =  (60 · 3/25) tree/hr  =  7.2 tree/hr