Assessing the bending strength of a wooden roof. The white wood material used for the roof of an ancient Japanese temple is imported from Northern Europe. The wooden roof must withstand as much as 100 centimeters of snow in the winter. Architects at Tohoku University (in Japan)

conducted a study to estimate the mean bending strength of the white wood roof (Journal of the International Association for Shell and Spatial Structures, Aug. 2004).

A sample of 25 pieces of the imported wood was tested and yielded the following statistics on breaking strength (in MPa): x = 75.4, s = 10.9.

a. Estimate the true mean breaking strength of the white wood with a 90% confidence interval. Interpret the result.

b. Suppose you want to estimate the true mean breaking strength of the white wood to within 4 MPa, using a 90% confidence interval. How many pieces of the imported wood need to be tested?