Answer: (36.7, 39.3)
Explanation:
Confidence interval for population mean:
, where n= sample size,
= sample mean,
= population standard deviation,
= critical z value.
Given: n= 40 ,

Critical z-value for 90% confidence = 1.645
Then, required confidence interval:

Hence, a 90% Confidence Interval = (36.7, 39.3)