@kwiley in case you don't realize how to do the math they just did, I'll outline it here quickly. That's a total of 5 gallons. 4/5 gallons is 0.8 (80%), 1/5 gallons is 0.2. So it's 0.8*1.060 + 0.2*1.000 = 1.048.
If you instead had, say, 4 gallons of juice at 1.060, and you topped up with 1/2...