Solve the Triangle Symbol Equation: △ - 9 = 20

To solve this problem, we'll follow these steps:

• Start with the equation: $\triangle - 9 = 20$
• Add 9 to both sides to isolate $\triangle$.

Now, let's carry out these steps:

Starting with the equation $\triangle - 9 = 20$.
To find $\triangle$, we add 9 to both sides:
$\triangle - 9 + 9 = 20 + 9$

Simplifying this gives us:
$\triangle = 29$

Therefore, the numerical value represented by $\triangle$ is $\boxed{29}$.

Looking at the multiple-choice options, we find that $29$ matches choice 3.