Total number of mandays required = W
W/5 - W/6 = 8
Multiply 30 on both sides:
6W - 5W = 240
W = 240
Total additional number of workers to hire to complete work 28 days earlier = N
240/5 - 240/(5+N) = 28
48 - 240/(5+N) = 28
48(5+N) - 240 = 28(5+N)
240 + 48N -240 = 140 + 28N
20N = 140
N = 7

But in real life, we all know that usually the total number of days will not reduce even if more people "work" on it. There will just be more people using "eye power" :)