For the second triangle (middle):

Let's find the pattern in the given triangles. We will denote the numbers on the two bottom sides as $S_1$ and $S_2$, and the internal number as $I$.

• For the second triangle (middle): The numbers on the bottom sides are $S_1 = 5$ and $S_2 = 5$. The internal number is $I = 15$. Let's test the pattern: $I = S_1 + S_2 + S_1$.

  $$I = 5 + 5 + 5 = 15$$

  This pattern works for the second triangle.

• For the third triangle (bottom, based on previous context): The numbers