how can i find the mass of oxygen in Potassium chlorate?

Question

for instance my schoo did a lab, which we heated in a crucible with cover and potassium chlorate and we had to find the masss of the cricible and cover which was 18.1g, and the mass of crucible,cover, and contents before heateing-19.94g and Mass of crucible cover and contentes AFter heating -19.8g

Answer 1

Mass of oxygen = 19.94 - 19.8

Mass of original potassium chlorate = 19.94 - 18.1

Now calculate the fraction of oxygen.