Suppose 6

Suppose the life span of a revolutionary light bulb is normally distributed with a mean life span of 70 thousand hours and a standard deviation of 3 thousand hours. If a light bulb is chosen at random:

a) what is the probability the life span will be within 5 thousand hours of the mean;
b) what must its lifetime be if it is to be categorized in the tallest 5%?

Correct answer:

p =  0.9522
t2 =  76000 h

Step-by-step explanation:

m=70000 h σ=3000 h  p=N(t>m+5000,m,σ) z1=σ75000m=30007500070000=35=1321.6667 p=N(z>z1,0,1)  p=0.9522

Use the normal distribution table.
t2=m+2 σ=70000+2 3000=76000 h



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