Arithmetic calculated fields combine numeric operands. Start by identifying the units: an annual salary divided by twelve gives a monthly equivalent, while annual salary divided by FTE gives a salary-per-FTE figure under this exercise's definition. Those measures answer different questions even when both are currency amounts.
Use a field reference when the amount changes by worker and a constant when the number is fixed. A value typed from one worker's current salary will fail as soon as another worker or a hidden tenant has different pay. Division by zero raises an error by default. Return Zero on Error suppresses that error and returns zero when you explicitly select it. The exercises include that boundary explicitly so you can inspect the result before using it in a report.