Given details are,
Initial number of workers are = 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
By using the formula,
A = P (1 + R/100)
x = 6400 (1 – 25/100) (1 – 25/100) (1 + 25/100)
= 6400 (75/100) (75/100) (125/100)
= 6400 (0.75) (0.75) (1.25)
= 4500
∴ Number of workers working during the fourth year is 4500.