Let's imagine we have a mystery number, let's call it t. We also have some other numbers in our equation: 19, 72, and 81. Now, the equation looks like this: 19 + (72 + t) = (19 + 72) + 81. Our goal is to figure out what the mystery number t is.
First, we need to simplify the equation by solving what is inside the brackets. So, we add 72 and t together to get 72 + t. The equation becomes 19 + (72 + t) = 91 + 81.
Next, we add the numbers outside the brackets to simplify the equation further. 19 plus 72 is 91. So now the equation looks like this: 91 + t = 172.
Finally, to find out what t is, we need to isolate it. We do this by subtracting 91 from both sides of the equation. This leaves us with t = 81.
So, the mystery number t is 81. By following these steps and simplifying the equation, we were able to solve for t.