Answer:
600 soldiers
Step-by-step Explanation:
Let's assume that the number of soldiers who joined the fort after 13 days is x.
After 13 days, the total number of soldiers in the fort is 1500 + x.
We know that the food in the fort lasts for 25 days after 13 days.
This means that the total number of days of food left after 13 days is 25 × (1500 + x).
We also know that the total number of days of food available is 1500 × 48 = 72,000.
Therefore, we can set up the following equation:

Divide both sides by 25.


Subtract 1500 on both sides:
Solving for x, we get:
x = 600
Therefore, 600 soldiers joined the fort after 13 days.