17.2.Lifetime (in hours) of a device is described by a r. variable with Normal distribution with parameters µ = 200 hours, variance σ2 = 400. What is the mean lifetime?

Further compute, with the help Excel (or of table of N(0,1) distribution function)

1. probability that device survives 180 hours;

2. such time that 90% of devices survive it (i. e. compute the 10% quantile of the distribution);

Write the density function of this normal distribution.