Let there be $'x'$ men in the beginning so that after 15 days the food for them is left for 45 days.
After adding 500 men the food lasts for only 40 days.
Now $(x+500)$ men can have the same food for 40 days.
Therefore by equating the amount of food we get,
Therefore there are 4,000 men in the fort.
Error(s) Found !!!
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