Construct a 90% confidence interval in the form estimate+_ margin of error

Question

In a survey 21 people were asked how much they spent on their child last birthday gift.The results were roughly bell shaped with a mean of $40a and standard deviation of $19 . Construct a 90% confidence interval in the form estimate +_ margin of error

Answer 1

CI = estimate +/- margin of error

estimate is 40

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

alpha = (100-90)*/100 = 0.1. t-critical for alpha/2 = 0.05 and sample size - 1 = 20 degrees of freedom = 1.725

margin of error = 1.725 * 19/sqrt(21) = 7.15

CI = 40 +/- 7.15