To help with thinking about this, we can say that the 6 gallons of 50% acid contains 3 gallons of pure acid and 3 gallons of filler. This would make the concentration 3/6, or 50%. Adding a gallon of pure acid would shift the concentration to 4/7, or 57.1%.
When the relationship becomes 70%, four gallons of pure acid have been added.
To buy four tickets it would cost $217.5 because C(x) = 49.5(4) +19.5 C(x) = 198 +19.5 C(x) = 217.5 With $465 you could buy 9 tickets. C(x) = 49.5(9) +19.5 C(x) = 445.5 + 19.5 C(x) = 465