Final answer:
The borders of legislative districts in Washington are determined by the number of people living in the district.
Step-by-step explanation:
The number of people living in the district determines the borders of each legislative district in Washington.
Population is determined every 10 years with the census, which helps establish how many persons live in the United States and where. Redistricting occurs after the census to maintain similar numbers of voters in each district while still maintaining a total number of 435 districts.
By law, electoral districts must have roughly the same number of people in each equivalent district, so the boundaries of legislative districts are redrawn as needed to achieve this balance.