Question 5865
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 {{{1/35}}} of the job
each person can perform {{{1/20 of the daily work, or (1/35)*(1/20)=(1/700)}}}
after 11 days:
{{{11*20*1/700 = 220/700}}}
4 days later:
{{{4*16*1/700 = 64/700}}}
There is now {{{285/700}}} work complete, leaving {{{415/700}}} work.
If we divide 415/700 by 20, it will give us the amount of work each day that must be completed: {{{20.75/700}}}
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
{{{560/700 + 44/700 + 100/700 = 704/700}}}