The simple example above is for a task considered to take a total of 15 man days.
This means it would take one person 15 days to complete or 15 people one day, if it was just a matter of effort.
However, the availability of people is not always reliable.
For the above case if you have 4 people available for the job it should take 15/4 = 3.75 days to complete.
However, if they are only available for 75% of the time you only have, in effect, 4x75% = 3 people.
Thus, according to the calculation shown, the job will take 5 days.