Research has found that the average number of suspended particles of air in a particular city is currently 80 micrograms per cubic metre. Suppose city officials work with businesses, commuters and industries, and introduce initiatives to try to reduce this figure. These officials hire an environmental company to take random measurements of suspended particles, in micrograms per cubic metre of air, over a period of several weeks. The resulting data follow. Take the population standard deviation to be 10.25 micrograms per cubic metre of air. Use these data to determine whether the city?s average number of suspended particles is significantly lower than it was when the initial measurements were taken. Let ? = 0.05.

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