How did they find that the two-tail area is 0.1? I don't understand can you explain how they came up with this?

GUIDED PRACTICE 7.10 The FDA's webpage provides some data on mercury content of fish. Based on a sample of 15 croaker white fish (Pacific), a sample mean and standard deviation were computed as 0.287 and 0.069 ppm (parts per million), respectively. The 15 observations ranged from 0.18 to 0.41 ppm. We will assume these observations are independent. Based on the summary statistics of the data, do you have any objections to the normality condition of the individual observations? EXAMPLE 7.11 Estimate the standard error of a = 0.287 ppm using the data summaries in Guided Practice 7.10. If we are to use the t-distribution to create a 90% confidence interval for the actual mean of the mercury content, identify the degrees of freedom and ta The standard error: SE - 0.069 V15 - 0.0178. Degrees of freedom: df = n - 1 = 14. Since the goal is a 90% confidence interval, we choose th, so that the two-tail area is 0.1: thy = 1.76. 3The sample size is under 30, so we check for obvious outliers: since all observations are within 2 standard deviations of the mean, there are no such clear outliers