Let's assume
Number of days needed after 25 days is x days
1200 men have food for 80 days
so, total men-foods is

but at the end of day 25, 320 men retire
So, total men-foods in 25 days is

total men-foods after 25 days is

now, we can equate them

now, we can solve for x




Hence,
Number of days needed after 25 days is 75 days...........Answer