The correct probability of putting kitchen waste = the amount of kitchen waste/total amount =400/(400+30+20)=8/9.

(2) Analysis: ∵ In the "kitchen waste" box, littering by mistake =30+20=50; in the "recyclable" box, littering by mistake = 100+20= 120; in the "other garbage" box, littering by mistake.

∴ The total amount of garbage thrown by mistake is 50+ 120+ 130=300.

∴ Probability of wrong throw of domestic garbage = total amount of wrong throw/total amount of garbage =300/ 1000=3/ 10.

(3) Analysis: Assume that the amount of kitchen waste in the "kitchen waste" box, "recyclable" box and "other waste" box is A, B and C respectively. As can be seen from the table, a+b+c=600. When the variance of data A, B and C is the largest, that is, when the kitchen waste is completely placed correctly, a=600.