The lifetime of a special type of battery is a random variable with mean 40 hours and standard deviation 20\(^\dagger\). hours A battery is used until it fails, at which point it is replaced by a new ne. Assuming a stockpile of 25 such batteries, the lifetimes of which are independent, approximate the probability that over 1100 hours of use can be obtained.
For the convenience, I will slightly deviate from the numbers in the question
Essentially the idea is to use the mean and the standard deviation in the central limit theorem to make calculations on the probability. For applying the central limit theorem, we need
Needed: \(P \{ X \ge 1100 \}\) where \(X\) is the sum of 25 independent and identically distributed exponential random variables
Using the central limit theorem we get \[P\{\frac{\overset{25}{\underset{i=1}{\Sigma}}x_i}{40\sqrt{25}} > \frac{(1100 - 1000)}{40\sqrt{25}}\} = 1 - \Phi(0.5) \approx 0.305338\]
We can also use simulation to see how closely it matches up to the calculated value. This should be a fairly simple
calculation and shown in the images below.
How to read the image?
DistributionChart[
Table[Table[Module[{data = Plus @@@ RandomVariate[ExponentialDistribution[1/40], {r, 25}]},
N[Length[Select[data, # > 1100 &]]/r]], 1000], {r, {100, 1000, 2000}}]
, ChartElementFunction -> "HistogramDensity"
, ChartLabels -> {100, 1000, 2000}]
Export[StringReplace[NotebookFileName[], ".nb" -> "_lifeTime_calculation.svg"], %]
ClearAll[lifeTimeMapper, lifeTimePoint];
lifeTimeMapper[list_List, r_: 0] := Module[{coordinates = {{0, r}}}, coordinates = coordinates~Join~({#, r} & /@ list)]
lifeTimePoint[list_, limit_: 1100] := {If[#[[1]] <= limit, Red, Green], PointSize[0.005], Point[#]} & /@ list;
Module[{data = Accumulate /@ RandomVariate[ExponentialDistribution[1/40], {1000, 25}]},
Graphics[{
{Thickness[0.005], Green, Line[{{1100, 0}, {1100, 10 Length@data}}]},
Parallelize[{Thickness[0], Line[#]} & /@ (MapThread[lifeTimeMapper, {data, 10 Range[Length@data]}])],
Parallelize[lifeTimePoint[#] & /@ (MapThread[lifeTimeMapper, {data, 10 Range[Length@data]}])]
}, Frame -> True, FrameTicks -> {True, None}]
]
Export[StringReplace[NotebookFileName[],".nb" -> "_lifeTime_image.svg"], %]