The ratio of the areas is 1200/350 = 24/7. So, the ratio of the paint volumes is 24/7, too. The number of units of magenta is 3 per each quart, or 3*4 = 12 units per gallon. Hence, the number of units of magenta is 24/7 times 12, or= = 41 for the mural of 1200 sq ft. Round it as you want.