Find the pattern to determine the internal number for the first triangle given the other two.

The problem presents three triangles with numbers on their sides and an internal number for the second and third triangles. We need to find the pattern to determine the internal number for the first triangle.

Let's denote the number on the left side as $L$, the number on the right side as $R$, and the internal number as $I$.

• For the second triangle: $L = 3$, $R = 5$, $I = 15$. Let's test two possible operations:
  1. Product: $L \times R = 3 \times 5 = 15$. This matches $I$.