In the past, a chemical company produced a certain type of plastic. Now, using a newly developed and less expensive process, the mean daily yield of plastic for the first 50 days of production is 871...

In the past, a chemical company produced a certain type of plastic.  Now, using a  newly developed  and less expensive process, the mean daily yield of plastic for the first 50 days of production is 871 pounds; the standard deviation is 21 pounds.

(a) Find the value of Z_a
/2in order to find a
90% confidence interval for the meandaily yield of plastic per day for the newly developed process.