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.