So start of by making the equation 500=120+20x. x is representing the number of weeks. Solve for x by subtracting 120 from both sides (380=20x). Now divide both sides by 20 (19=x). You know it will take 19 weeks; however, the question asks how many days. Since there are 7 days in a week, just multiple 7x19. The answer is 133 days.