A consumer advocacy group tested the "on-air" lifetimes of a random sample of 152 cell phone batteries. The mean lifetime was 2.9 hours with a standard deviation of 0.4 hours. The lifetimes are Approximately bell-shaped. Estimate the number of batteries with lifetimes between 2.5 hours and 3.3 hours.
A) 103
B) 144
C) almost all (greater than 144)
D) 49

Question

Quizplus · Accepted Answer

The z-scores for 2.5 hours and 3.3 hours are -1 and 1, respectively. The area under the normal curve between -1 and 1 is approximately 0.68. Therefore, the number of batteries with lifetimes between 2.5 hours and 3.3 hours is approximately 0.68 * 152 = 103.

A Consumer Advocacy Group Tested the "On-Air" Lifetimes of a Random