David owns a food truck that sells tacos and burritos. He sells each taco for $4.50 and each burrito for $7.75. Yesterday David made a total of $519.25 in revenue from all taco and burrito sales and there were twice as many tacos sold as there were burritos sold. Write a system of equations that could be used to determine the number of tacos sold and the number of burritos sold. Define the variables that you use to write the system.