I'm trying to pick up an old exam that's been left hanging,
needless to say my math is a bit rusty. I came across the
following problem and don't remember how to handle components
that already has a run time different from t0.  The problem at
hand: Two components that are coupled in series in a block
diagram have a ware-out distribution which is a Weibull
distribution with shape factor 2. The t0 = 1000 hours. The mean
time to failure for both is 3000 hours. One of them has run 1000
hours and the other 1500 hours. Calculate the probability for
the system to survive the next 10 days (=240 hours).  How is the
probability calculated for the component with the run time of
1500 h? The answer to the whole problem would also be deeply
appreciated!