60 points in 4.5 gallons is (60 * 4.5) 270 "gravity points" total. When you add a gallon of water to get to 5.5 gallons, you're not adding any points so you still have 270 gravity points but now it's in 5.5 gallons. So, you're new gravity is (270 / 5.5 = 49) 1.049.