An example of the basic scheduling behavior that you'll see when you change the Work on a Fixed-units task:
You assign 1 painter full-time (100%) to paint walls in 4 days (the duration). The work (the time it will take the painter to complete the task at 100% effort) is 32 hours, or 4 days.
If you increase the amount of work to 40 hours, Project recalculates the duration as 5 days. Now the painter has an extra day of work on the task.
If you assign another painter full-time, both painters, applying 100% of their time to the task, will get the job done in only 2 days.
Duration = Work ÷ Resource Units
Project gauges work in terms of resource assignment units—the percentage of available working time, based on the resource calendar, that a resource spends on an assigned task.
Resource assignment units in Project are expressed either in percentages or decimals. At 100% units, a resource is working full-time; at 50% units, the resource is working half-time, and so on.
The default setting in Project scheduling is Fixed Units, meaning that whatever resource units you allocate, that value will remain fixed and the other two elements of the scheduling formula will be affected by changes.