Given,
Intial number of workers = 6400
At the end of first year = 25% retrenched
At the end of second year = 25% retrenched
At the end of third year = 25% increased
So,
Number of workers working during fourth year
\(= 6400({1}-\frac{25}{100})({1}-\frac{25}{100})({1}+\frac{25}{100})\)
\(= 6400\times\frac{3}{4}\times\frac{3}{4}\times\frac{5}{4}\) = 4500 workers