x = worker days needed to complete the job
x/5 = days for 5 to complete
x/(5+1) = x/5 - 8
x/6 = x/5 - 8
5x/30 - 6x/30 = - 8
solve for x = 240 worker days
Now for 28 days less:
240/ ( 5 + n) = 240/5 -28 where n = number of additional workers
240/(5+n) = 20
n = 7 additional workers needed above the original 5