1. The problem is unclear as stated: "Why sum xy i dalat. 75.3". It seems to ask about the sum of a product $xy$ or a related concept involving the number 75.3. 2. To clarify, if you want to find the sum of products $xy$ for given values of $x$ and $y$, you need to know the values or the context (e.g., a series or data set). 3. If the question is about summing $xy$ where $x$ and $y$ are variables or numbers, the sum is simply $\sum xy$ over the given range or data. 4. Without more information, the best we can do is explain that summing $xy$ means adding all products of $x$ and $y$ pairs. 5. If 75.3 is a total or a value related to the sum, please provide more details for precise help. Please provide more context or clarify the question for a detailed solution.