ABS plastic is a polymer consisting of three monomer units: acrylonitrile (C₃H₃N), butadiene (C₄H₆), and styrene (C₈H₈). A sample of ABS plastic contains 12.9% N by mass. It took 1.56 g of Br₂ to react completely with a 2.00 g sample of ABS plastic. Bromine reacts 1:1 (by moles) with the butadiene units in the polymer and nothing else, so bromination is a method for determining the quantity of butadiene in the polymer. What is the percent by mass of styrene in the polymer?