If 6.0 L of antifreeze solution (specific gravity = 0.80) is added to 5.0 L of water to make a 11 L mixture, w

Answer 1

... what's the specific gravity of the mixture?
Since the total volume is the sum of the two ingredients, there was no volume loss due to mixing. (Sometimes there is; water and ethanol are an example.) Then the total mass is 6*0.8 + 5 * 1 = 9.8 kg and the sp. gr. = 9.8/11 = 0.8909 kg/L.