In a workplace with 300 employees, the following occurence of new cases of respiratory distress was observed over three months:...

In a workplace with 300 employees, the following occurence of new cases of respiratory distress was observed over three months: end of month 1: 15 cases; end of month 2: 10 cases; end of month 3: 5 cases. For calculation of the incidence rate which of the following values for person-months should be used in the denominator?

1) 500 person-months

2) 300 person-months

3) 320 person-months

4) 860 person-months