Lets say for example, that a certain business X is undertaking a new project:

In year 1 the business is attracting 100 customers

In year 2 110 customers

In year 3 121 customers

In order to proceed with the project. The business would need additional staff salaries. Those would be 10k in year 1, 20k in year 2 and 30k in year 3, in addition of existing annual staff salaries of 15 k. are the additional staff salaries a fixed cost, a variable cost, or a semi-fixed cost? In the last case, how de we calculate it ?