Question

35.8g of a mixture of potassium chloride and potassium trioxochlorate(v), we're heated to a constant mass. If the residue weighs 24.9g.what was the percentage mass of the potassium chloride in the mixture (k=39, cl=35.5, o=16)

Answer 1

I assume you mean potassium chlorate instead of that screwy name you have for KClO3. The idea behind this problem is that the KClO3 decomposes to oxygen and KCl. The difference between initial and final weight will be the loss in O2. Convert that to mass KClO3 initially. Then difference between mass KClO3 and initial mass is mass KCl.
%KCl = (mass KCl/35.7)*100 = ?
Post your work if you get stuck.
Decomposition KClO3 is
2KClO3 ==> 2KCl + 3O2

Answer 2

%KCl =24.9÷35.7=0.6974×100=69.74%