Problem based on Stoihiometry

A sample of polystyrene prepared by heating styrene with tribromobenzoyl peroxide in the absence of air has the formula Br_3, C₆H₃(C₈H₈)n. The number n varies with the condition of preparation. One sample of polystyrene prepared in this manner was found to contain 10.46 % bromine. What is the value of n? Ans.. n = 19