Lincoln has set up a lemonade stand outside his house and sells small cups and large cups of lemonade. Each small cup holds 6 ounces of lemonade and each large cup hold 18 ounces of lemonade. Lincoln used 780 ounces of lemonade and sold 4 times as many large cups as small cups. Write a system of equations that could be used to determine the number of small cups sold and the number of large cups sold. Define the variables that you use to write the system.