The plasma membrane, or the cell membrane, provides protection for a cell. It also provides a fixed environment inside the cell. And that membrane has several different functions. One is to transport nutrients into the cell and also to transport toxic substances out of the cell.
The substage condenser is either incorrectly positioned or the light stop is the wrong size for the objective- or there me be a smudge/mark on the eyepiece
The expression would be $0.20 x f. Once you figure out what the 'f' variable stands for, you have to multiply it by $0.20, and you will have your answer.
