Question 805112
20 people working 30 days are 600 man work days to finish the job.

If it were 5 days it would be
20*5=100 man work days.
15 people working for 30 more days
15 * 30=450 man work days
100+450=550
that only adds up to 550 man work days which isn't enough to finish the job.
The correct answer is
20x+15y=600,
x+y=35
x = 15,   y = 20
5 people leave after 15 days 
and 15 people finish the work in 20 more days