1 gal (initial) = (5/6) parts fuel & (1/6) parts antifreeze= total of 6 parts
0.5 gal (add'l) = (2/3) parts fuel & (1/3) parts antifreeze = total of 3 parts
adding the two
Fuel: 1gal*(5/6 parts) + 0.5gal*(2/3 parts) = 7/6 parts in 1.5 total gallons
Antifreeze: 1gal*(1/6 parts) + 0.5gal*(1/3 parts) = 2/6 parts in 1.5 total gallons
thus with a common denominator provides a ratio of 7:2 (fuel:antifreeze)