A 2.60 kg steel gas can holds 15.0 L of gasoline when full. What is the average density of the full gas can, taking into account the volume occupied by steel as well as by gasoline?

Question seems to be incomplete without density of gasoline and steel. Assuming density of gasoline to be 0.8 kg/lit and of steel to be 7.8kg, total weight=2.6+(15x0.8)=14.6 kg

Total vol=2.6/7.8+15=0.33+15=15.33 lit
Av density=14.6/15.33=0.95 kg/lit