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
trees per hour = tree/hr

Since the unit of time in the data (minutes) is different from the unit of time in the answer (hours, with any partial hour in minutes), we will also have to convert the time unit using the identity

60 min  =  1 hr