To determine the efficiency of a batch of electric motors, 10 motors are tested. The average efficiency of the sample is calculated to be 91.0% and...

To determine the efficiency of a batch of electric motors, 10 motors are tested. The average efficiency of the sample is calculated to be 91.0% and the standard deviation of the sample is 0.8%. If we want to cut the confidence interval by one-half, how many more motors should be tested?