Explanation

fort had provision of food for 150 men for 45 days,
After 10 days 25 men left the fort, so 125 men are there in the fort.
So, now fort is having food for 150 men for 35 days.

Suppose 125 men had food for x days.

Now, Less men, More days (Indirect Proportion)

125×x=150×35

x=42days