Final answer:
The critical value z_c necessary for a confidence interval at a confidence level of c = 0.87 is approximately 1.136.
Step-by-step explanation:
The critical value z_c necessary for a confidence interval at a confidence level of c = 0.87 can be found using the standard normal probability distribution table or a calculator. The critical value is the z-score that corresponds to the desired confidence level. In this case, c = 0.87.
Using a table or calculator, we can find that the critical value z_c for a confidence level of c = 0.87 is approximately 1.136.