Answer:
50 people
Explanation:
Given a research project that take 16 people 125 days to complete, you want to know the number of people needed to complete the project in 40 days.
People-days
In math, project effort is measured in people×days. That value is considered to be a constant for a given project. This makes the number of people inversely proportional to the number of days, and vice versa.
(16 people)×(125 days) = 2000 people·days = (p people)×(40 days)
Dividing by 40 days, we have ...
p people = (2000 people·days)/(40 days) = 50 people
50 people are needed to complete the research in 40 days.
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Additional comment
In real life, more people may get in each other's way. Or too few people may cause motivation, cooperation, and synergy to be lost. The maxim, "adding people to a late project makes it later" has a certain basis in reality.