Explanation:

Assume x soldiers join the fort. 1200 soldiers have provision for 1200

(days for which provisions last them)

(rate of consumption of each soldier)
= (1200)(30)(3) kg.

Also provisions available for (1200 + x) soldiers is (1200 + x)(25)(2.5) k

As the same provisions are available
= (1200)(30)(3)

= (1200 + x)(25)(2.5)

x = ([(1200)(30)(3)] / (25)(2.5)) - 1200

x = 528.

Hence, option 3 is the correct answer.