How many days does each person contribute to the total project days?
Individuals don't contribute to the project's time. A can do a project, alone, in a days, but a does not increase the time of a project. Rather, 1/a contributes to the reciprocal of the project's time.
1/a +1/b+1/c = 1/10, etc.
Allow me to rephrase cause I disagree: Including someone on a project and excluding someone else directly affects the amount of days a project takes. So in terms of making a three man team we can determine the day load attributed to each person (as well as rank them by productivity). In terms of proportional pay for effectiveness this is very important as we in manufacturing projects pay individuals incentives based on their individual contribution to group projects.