An energy-saving lamp lights up an average of 10,000 hours before it fails, with a standard deviation of 800 hours.
Which lighting duration is not exceeded by 80% of the lamps?
My approach would be to take the Gaussian distribution
$p = \frac{1}{\sqrt{2 \pi \sigma^2}}e^{-\frac{(x - \mu)^2}{2 \sigma^2}}$
and transform to $x$ where $p = 80$.
Are there any other approaches?