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

Using this value, we can find the total time to pick 40 trees

r  =  x / t
7.2 tree/hr  =  (40 trees) / t
t  =  (40 trees) / (7.2 tree/hr)  =  (40/7.2) hr  ≈  5.55556 hr  =  5 hr + (0.55556 hr)(60 min/hr)  ≈  5 hr + 33 min  =  5 hr 33 min

Although I've written 5.55556 in the solution above, you should use a calculator and keep the number in memory or in the display with all of the digits. You should not actually round your result until you get to the final step of rounding to the nearest minute.