Answer:
Darell must make 20 trips to total 28 kilometers of cycling.
Explanation:
Given that Darell has already cycled 8 kilometers this year, plus he plans to cycle 1 kilometer during each trip to work, to determine how many trips he must make to total 28 kilometers of cycling, the following calculation must be performed: 
(28 - 8) / 1 = X 
20/1 = X 
20 = X 
Thus, Darell must make 20 trips to total 28 kilometers of cycling.