| Decision:
(i) Reject H0 if z z =1.645 if =0.05. z =28.57 > 1.645 and reject H0.
(ii) P-value = P(Z>28.57)=0. Since the P-value =0.05, reject H0.
Conclusion: the data provide compelling evidence that the true average strength for the 1078 grade exceeds that for the 1064 grade by more than 10.
Or you can answer the same question using  versus 
(b) 95% confidence interval for  is  = =(-16.84 , -15.16)
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