Question 108786
Since you've assumed a normal distribution, you can 
calculate your z value using the value you're intereted in, the mean and the  standard deviation.
{{{z=(x-x[m])/(sigma)}}}
where x = 8 years (your warranty date)
{{{z=(8-8.75)/(.45)}}}
{{{z=-1.667}}}
Your z score then gives you the percentage of units that failed before 8 years, that is, did not meet the guarantee.
At z=-1.667, the cumulative distribution function (from EXCEL) gives a value of 0.04779. 
In other words, from 0 to 8 years, the area under the curve is 4.8% of the total area. 
4.8% of the units failed to meet the guarantee.