Calculate days needed for the 4 workers to complete an 80 person-months job 
The workload required to develop a system is estimated at 80
  person-months if it is carried out by one person. When four staff
  members with the same productivity work together to develop the
  system, each member’s productivity is expected to decrease by 20%. How
  many working days does it take for the four members to develop this
  system? Here, this development work can be equally divided into four
  parts, and the four members can perform each individual part in
  parallel. In addition, each member can work 22 working days per month.

80 person-months is : $\cfrac{1}{80}$ 
So for the 4 of them we have : $0.8*t*\cfrac{4}{80} => t = \cfrac{80}{3.2} = 25$ 
And the result is : $ 25 * 22 = 550 $ days 
But the correct result is 528, what am I doing wrong here? Please show me, thanks alot! 

If each worker's productivity is cut by 20%, that means the total amount of work is $80 + .2*80 = 96$.  Thus, there are a total of $96*22/4 = 528$ working days.
