Celinda Corporation is a plumbing service provider in Melbourne. Over the past 4 years, the company mean value per sales invoice was $240. As the chief accountant, you need to inform the finance manager if this amount changes. Hence, you have gathered a sample of 16 invoices. According to that, the mean is $210 and the standard deviation is $45. If the population of the sales invoice is normally distributed, assess whether there is any change in mean sales (Sig level 5%). Note: You are required to use the 6 steps process in hypothesis testing.