13.78 Over the past year, an audio store has sold stereo systems produced by three different manufacturers. Of the 400 units from company A, 12% were returned for service during the warranty period. This compares to 15% of 300 units from company B, and 16% of 500 from company C. Assuming that the store’s sales are a representative sample of the stereo systems produced by the three manufacturers, use the 0.025 level of significance in testing whether the population percentages of warranty service returns could be equal.