An engineer is creating a new metal alloy by mixing Alloy A and Alloy B. Alloy A contains 80% copper by weight, and Alloy B contains 30% copper by weight. If the mixture contains a total of 500 kilograms of copper, which equation models this relationship, where $x$ is the weight of Alloy A in kilograms and $y$ is the weight of Alloy B in kilograms?