Joan's nursery specializes in custom-designed landscaping for residential areas. the estimated labor cost associated with a particular landscaping proposal is based on the number of planting trees, 1 shrubs, and so on to be used for the project. for cost-estimating purposes, managers use two hours of labor time for planting of a medium-sized tree. actual times from a sample of 10 plantintings during the past month follow (times in hours):