Answer:
To find the critical value tc for c = 0.90 and n = 15, we need to use a t-distribution table or calculator.
Using a table or calculator, we find that the critical value tc for a one-tailed test with a degree of freedom of 14 and a confidence level of 0.90 is approximately 1.761.
Therefore, if we have a sample of size 15 and want to perform a hypothesis test with a confidence level of 90%, we would reject the null hypothesis if our calculated t-value is greater than 1.761 or less than -1.761.
Explanation: