Final answer:
130 hamburgers were sold on Friday.
Step-by-step explanation:
To find the number of hamburgers sold on Friday, we can set up an equation. Let number of hamburgers sold is x. The number of cheeseburgers sold would then be 3x, since the number of cheeseburgers sold is three times the number of hamburgers sold. The total number of hamburgers and cheeseburgers sold is 520, so we can write the equation as x + 3x = 520. Combining like terms, we get 4x = 520. Dividing both sides of the equation by 4, we find that x = 130. Therefore, 130 hamburgers were sold on Friday.