Life expectancy

The life expectancy of batteries has a normal distribution with a mean of 350 minutes and a standard deviation of 10 minutes. What is the range in minutes 68% of the batteries will last?
What is the range in minutes? How long will approximately 99.7% of batteries last?

Correct answer:

a1 =  340 min
a2 =  360 min
b1 =  320 min
b2 =  380 min

Step-by-step explanation:

m=350 min σ=10 min  a1=m1 σ=3501 10=340 min
a2=m+1 σ=350+1 10=360 min
b1=m3 σ=3503 10=320 min
b2=m+3 σ=350+3 10=380 min

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