- CFA Exams
- CFA Level I Exam
- Study Session 3. Quantitative Methods (2)
- Reading 9. Common Probability Distributions
- Subject 8. The Standard Normal Distribution

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**CFA Practice Question**

The lifetime of a 2-volt non-rechargeable battery in constant use has a normal distribution with a mean of 516 hours and a standard deviation of 20 hours. Ninety percent of all batteries have a lifetime less than ______.

A. 517.28 hours

B. 536.00 hours

C. 541.60 hours

**Explanation:**The z-value corresponding to the proportion 0.9000 is 1.28. We need to use the formula x = u + z σ, where z = 1.28, to get the correct answer, or x = 516 + 1.28 x 20 = 541.6.

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**User Contributed Comments**
5

User |
Comment |
---|---|

awlhoaln |
how to get z value is 1.28 ? |

johnsk |
from the z-table. |

danlan |
prob(z<1.645) = 95% and prob(z<1.28)=90%. Note 1.645 used for 5% significance level is for two tail and not one tail! |

dimanyc |
how to get z value for 95%? memorize all your 90%, 95% and 99% values for both both 1 and 2-tailed tests |

gaetmichel |
table will be provided at the exam |