finding mu when sigma is unknown

Question

In a survey, 26 people were asked how much they spent on their child's last birthday gift. The results were roughly bell-shaped with a mean of $31 and standard deviation of $6. Construct a confidence interval at a 99% confidence level.

Answer 1

2.58 x 6 = 15.84. that would give 31 plus or minus 15.84 divided by square root of the sample size

(28,34) is the 99% confidence interval for a sample of 26

sample mean plus or minus 15.84/sqr26 = 28 and 34