Therefore, the workload of two shifts working for one hour at the same time is (2/15)+(1/6) = 3/10.
Two shifts work at the same time, and the time required to complete the fertilization task is:1÷ (3/10) =10/3 (hours).
Therefore, the first class and the second class in the morning can be arranged to fertilize the fruit trees at the same time. This task can be completed within 4 hours, and the actual time is 10/3 hours.