How to calculate the user minutes?

A user minute is defined as “The amount of time a particular user is present in the conference”. For example: Let's assume three users John, Alice, and Bob are in a conference of 10 minutes duration. John and Alice were present from the beginning and Bob joined after 5 minutes

User minutes for three participants:

John  : 10 minutes

Alice  : 10 minutes

Bob    : 5 minutes

Totally chargeable user minutes: 25 minutes