Question 292349
There is insufficient information to answer the question.  I depends on how fast the fourth person can work.  Perhaps he/she can work twice as fast as the others?  Or only half as fast?
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If we make the BIG assumption that everyone works at the same speed, and no one can go faster or slower than another, then we can solve the problem.  But that is a BIG assumption.
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If 3 people can do the work in 4 hours, then logically 4 people working at the same speed can do the job in 3 hours.
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Looking at any work problem, you have to think about the sum of their labors = 1 job done to completion.
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If it takes 4 hr, then 1/4 of the work is done per hr.
If there are 3 people, then each is doing 1/12 of the work per hr.
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With 4 people, their aggregate work = 4*1/12 = 1/3 per hr.
So it takes them 3 hr.
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Done.