The balanced chemical reaction will be:
2KClO3 = 2KCl + 3O2
We are given the amount of potassium chlorate being burned. This will be our starting point.
400.0 g KClO3 1 mol KClO3/ 122.55 g KClO3) (3 mol O2/2 mol KClO3) ( 32.00 g O2/1mol O2) = 156.67 g O2
Percent yield = actual yield / theoretical yield x 100
Percent yield =115.0 g / 156.67 g x 100
Percent yield = 73.40 %