KClO3 is potassium chlorate. (Potassium perchlorate is KClO4)
Anyway, to get to your question:
25g of potassium chlorate is how many moles? You find out by dividing 25g by the formula weight of potassium chlorate. Once you have this number of mols, then you use the stoichiometry of the chemical reaction to give you the number of mols of your product. In your case, your reaction uses 2 mols of potassium chlorate to produce 3 mols of oxygen gas, so there is a 2:3 ratio of mols of potassium perchlorate used: mols oxygen gas generated. Once you calculate the number of mols of oxygen gas you have, then you use the formula weight of oxygen gas (32 g/mol) to give you the number of grams of oxygen that you generated.
I think that you might be misreading the question a little; potassium chloride won't decompose to give oxygen--only potassium chlorate will do that (in your equation).
Hope this helps.