The life of a watch battery is normally distributed with a mean of 950 days and standard

Question:

The life of a watch battery is normally distributed with a mean of 950 days and standard deviation of 40 days. Using the appropriate Excel functions (see Chapter 5), determine the following:

a. What fraction of batteries is expected to survive beyond 1,010 days?

b. What fraction will survive fewer than 900 days?

c. Draw a graph of the reliability function using Excel.

d. What length of warranty is needed so that no more than 10 percent of the batteries can be expected to fail during the warranty period?

 

Watch Battery Life-Reliability













Enter data only in the shaded cells












This spreadsheet is designed to calculate the probability of values equal to, or less than, a desired x value, 
given the mean and standard deviation of a normally distributed variable. It uses the cumulative normal distribution
Enter the mean of the distribution in shaded cell D8 and the standard deviation in shaded cell D9. below. 
Enter the desired X-value in shaded cell D10, below. The calculated z-value and probability will be seen in D11 and D12. 








Mean of distribution950
950


Std deviation of distribution40
40


Desired x-value1010
900


Calculated z-value1.50
-1.25


Probability of x, or less0.93319
0.10565











(X-axis)Probability Using




Desired x-valuesEquivalent - Z ValuesNORMS.DIST




790-4.000.00003




794-3.900.00005




798-3.800.00007




802-3.700.00011




806-3.600.00016




810-3.500.00023




814-3.400.00034




818-3.300.00048




822-3.200.00069




826-3.100.00097




830-3.000.00135




834-2.900.00187




838-2.800.00256




842-2.700.00347




846-2.600.00466




850-2.500.00621




854-2.400.00820




858-2.300.01072




862-2.200.01390




866-2.100.01786




870-2.000.02275




874-1.900.02872




878-1.800.03593




882-1.700.04457




886-1.600.05480




890-1.500.06681




894-1.400.08076




898-1.300.09680




902-1.200.11507




906-1.100.13567




910-1.000.15866




914-0.900.18406




918-0.800.21186




922-0.700.24196




926-0.600.27425




930-0.500.30854




934-0.400.34458




938-0.300.38209




942-0.200.42074




946-0.100.46017




9500.000.50000




9540.100.53983




9580.200.57926




9620.300.61791




9660.400.65542




9700.500.69146




9740.600.72575




9780.700.75804




9820.800.78814




9860.900.81594




9901.000.84134




9941.100.86433




9981.200.88493




10021.300.90320




10061.400.91924




10101.500.93319




10141.600.94520




10181.700.95543




10221.800.96407




10261.900.97128




10302.000.97725




10342.100.98214




10382.200.98610




10422.300.98928




10462.400.99180




10502.500.99379



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