arithmetic - Word problem with group dividing time evenly

I was asked this question in an interview and am unsure of the correct answer:

There are 5 people invigilating a test but only 4 of them are to be present at a time. The test is 3 hours. Each invigilator works the same amount of time. How long does each work?

I know it's between $\frac{3}{5}$ and $3$ hours.

Answer

If each works $h$ hours, then $5h = 4 \times 3$ (the left-hand side is the number of person-hours worked by considering each invigilator; the right-hand side is the number of person-hours worked by considering the exam hall at each instance of time). So $h = \frac{12}{5}$.