SOLUTION: Twenty laborers could finish a certain construction job in 35 days. After 11 days, five laborers quit the work and are not replaced until 4 days. How many more laborers should be a

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This is easiest if we define how much work each laborer can perform in a day:
20 laborers can do the entire job in 35 days, so each day 20 workers can perform of the job
each person can perform
after 11 days:

4 days later:

There is now work complete, leaving work.
If we divide 415/700 by 20, it will give us the amount of work each day that must be completed:
so they must hire an additional 5 people (20.75-16 = 4.75, then round up)to finish the job.
check by the amount of work performed:
16 people * 35 days * 1/700 = 560/700
4 people * 11 days * 1/700 = 44/700
5 people * 20 days * 1/700 = 100/700