estimating mu when sigma is unknown

Question

In a survey, 21 people were asked how much they spent on their child's last birthday gift. The results were roughly bell-shaped with a mean of $39 and standard deviation of $7. Find the margin of error at a 80% confidence level.

Give your answer to two decimal places

Answer 1

margin of error = critical t-score * standard deviation/square root(sample size)

for 80% confidence level and sample size = 21, critical t value is for 0.1 tail and 20 degrees of freedom = 1.325

so margin of error = 1.325 * 7/sqrt(21) = 2.02