On basis of extensive tests, the yield point of specific kind of mild steel-reinforcing bar is known to be normally distributed with ? = 100. The composition of the bar has been slightly modified, but the modification is not believed to have affected either the normality or the value of ?. Assuming this to be the case, if a sample of 36 modified bars resulted in a sample average yield point of 8,439 lb., provide the 90% CI for the true average yield point of the modified bar.