Question 4 (10 marks)

Celinda Corporation is a plumbing service provider in Melbourne. Over the past 4 years 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 12 invoices. According to that the mean $230 and the standard deviation is $44.

If the population of sales invoice is normally distributed, assess whether there is any change in mean sales (Sig level 5%).