Question

The lifetime of a certain brand of lightbulbs is normally distributed with the mean of 3800 hours
and standard deviation of 250 hours.

The probability that randomly selected lightbulb will have lifetime more than 3500 hours is ________

The percent of lightbulbs which have the lifetime between 3500 and 4200 hours is __________

What lifetime should the manufacturer advertise for these lightbulbs if he assumes that 10%
of lightbulbs with the smallest lifetimes will burn out by that time?

Advertised time is ______ hours.

Answer 1

a).The probability that randomly selected lightbulb will have lifetime more than 3500 hours is:-

[ using standard normal table ]

b).The probability of lightbulbs which have the lifetime between 3500 and 4200 hours is:-

[ using standard normal table ]

The percentage of lightbulbs which have the lifetime between 3500 and 4200 hours is 91.04 %

c).Let a be the advertised time.

according to the problem,

[ in any blank cell of excel type =NORMSINV(0.1) ]

Advertised time is 3543.69 hours.

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