Introduction:
To find the third side of the given numbers, we need to understand what type of triangle we have and what formula we can use to calculate the missing side.

Given Numbers:
15, 10, 20, 25, 18, 11, 9, 12

Type of Triangle:
To determine the type of triangle, we need to check if the sum of the two smaller sides is greater than the largest side.

In this case, the largest side is 25.

Sum of the two smaller sides:
10 + 9 = 19
12 + 11 = 23
15 + 18 = 33
20 + 11 = 31

As we can see, only the sum of 15 and 10 is less than 25. Therefore, we can conclude that the given numbers do not form a triangle.

No Third Side:
Since the given numbers do not form a triangle, there is no third side to calculate.

Conclusion:
It is important to understand the properties of triangles and how to determine the type of triangle based on the given sides. In this case, we can see that the given numbers do not form a triangle, and therefore, there is no third side to calculate.