Sophia wrote an equation to represent the revenue of a movie theater for one day. She let x represent the number of child tickets sold and y represent the number of adult tickets sold. If a child ticket costs $4, an adult ticket costs $6, and the total revenue for the day was $420, which equation could Sophia use to represent the number of child and adult tickets that were sold?
6 x + 4 y = 420
4 x + 6 y = 420
4 x + 6 y = 840
6 x + 4 y = 840