What do you mean by an additive quantity?
Additivequantity has the property that the function of the union or sum of two quantities is equal to the sum of the functional values of each quantity. like heat is an additive quantity if we consider 40J of heat energy in a system or we add two systems having energy 20J, the effect is same in both cases.so it is an additive quantity.
but on the other hand if we add two systems having tempearture 50oC then temperature of overalll system would be 50oC not 100oC. so temperature is not an additive quantity.