Sorry if this answer comes rather late.
The specific gravity of chemical A is 1.2 means it's 1.2 times as dense as water; in other words, if you have equal volumes of water and chemical A, the mass of chemical A is 1.2 times the mass of the water. A litre of water has a mass of 1 kg, so if you mix 11 litres of water and 5 litres of chemical A, the total mass is
And the volume is litres.
So the specific gravity of the mixture is
I hope that helps.