A completion time tells you how long a whole job takes. A work rate tells you how much gets done in one unit of active time. The difference matters: when two people work together, their contributions add, but their solo completion times do not. By the end you will be able to name the whole job and rate units, calculate a rate from partial work, combine constant independent rates, recover a missing contributor, and compare efficiency with the time needed for the same job. You will also check whether the supplied information permits the claimed answer. This lesson retrieves exact fractions, equivalent ratios and inverse proportion from Numbers and Ratios. “Efficiency” here means completed work per unit active time. It does not mean work quality, physical effort, personal worth or energy efficiency. Changing teams and tank flow come in the next two lessons.