Answer: C. 541.60 hours
Explanation:
Since the lifetime of a 2-volt non-rechargeable battery in constant use has a normal distribution, we would apply the formula for normal distribution which is expressed as
z = (x - µ)/σ
Where
x = lifetime of the batteries.
µ = mean lifetime
σ = standard deviation
From the information given,
µ = 516 hours
σ = 20 hours
Looking at the normal distribution table, the z score corresponding to ninety percent approximately is 1.28
Therefore,
1.28 = (x - 516)/20
Cross multiplying by 20, it becomes
20 × 1.28 = x - 516
25.6 = x - 516
x = 516 + 25.6
x = 541.6
Therefore, Ninety percent of all batteries have a lifetime less than
541.60 hours