Final answer:
Rita's first mistake was in setting up the equation. In mathematics, two unequal numbers cannot be stated as equal. Therefore, the statement '12 equals 18.3' is inherently wrong.
Step-by-step explanation:
In the situation presented, Rita attempted to equate two distinct numbers, 12 and 18.3. Her first mistake took place in the setting up of the equation. In mathematics, you can't state that two different numbers are equal. For instance, saying that 12 equals 18.3 is inherently incorrect because it does not correspond to the factual numerical values in question. Hence, the error doesn't lie in the calculation process since a calculation wasn't performed. It's essential to properly set up equations in mathematics to ensure they make logical sense and can indeed be solved.
Learn more about Setting up equations