The probability of the disk lasting less than 3 years is very high.
Considering that on average it's around 4 years with a standard devaiation of 0.8
M = 4
SD = 0.8
The probability that it will last less than 3 years means that
we have to substract 4 - 0.8 (the difference of 1 standard deviation is 34% of the data and the difference of 2 standard deviations is 14% of the data)
Because it's more than 3 years (3.2) we have to do this again.
3.2 - 0.8
is
2.6 - this is less than 3 years.
Now we see that in order for a disk to last less than 3 years, it has to be 2 standard deviations apart from the mean.
This means that there is a 5% probability of its breaking down.
