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Problem 3. We destroy a number of concrete beams by applying a large tensile load. Each time, we write down the tensile strength of the beam from the test. We obtain the following data: 363 376 376 377 380 382 386 389 392 394 401 402 403 408 409 418 420 Assume that the tensile strength has a normal distribution, but we don't know the true mean or variance. Form a twosided interval that you are 95% sure includes the population mean. Problem 6. Find a 95% confidence interval for the true standard deviation of the strength of these beams