Researchers want to study noise levels produced by vehicles on a busy street. Assuming the variable is approximately normally distributed, the mean noise level for 19 vehicles was 85.6 decibels, with a standard deviation 4.5 decibels.

a) What distribution should be used, z or t? Type z or t in the answer box. [1 mark] b) Find the critical Value for the 90% confidence interval. Write exact value from the table. [2 marks]

c) Find margin of error rounded to two decimal places. For example, 2.22 [1 mark]

d) Find the 90% confidence interval of the true mean. Round answers to two decimal places and separate the lower and upper limits of the interval with a comma and no space. For example: 10.13,12.64 [1 mark]