11-27. Two brands of comparable electric drills (call them A and B) are each guaranteed for 1 year under normal use. For n1 = 45 brand A drills, 10 failed to operate in under 1 year and for n2 = 48 brand B drills, 16 failed before the warranty period ended. Determine a 99% confidence interval for the true difference between the failure proportions during the warranty period. How precisely have we estimated μ1 − μ2?