A metal fabrication shop would like to know what the learning rate is for a certain section of the plant that makes welded assemblies. In one case study, a total of four assemblies were completed. Records were kept for units 1 and 4, and the times were 60 hours and 48.6 hours, respectively. Unfortunately, the times for units 2 and 3 were lost. Use the Crawford learning curve model to determine the learning rate indicated by the data?