2. During the transportation of a box containing 21 standard and 10 non-standard parts, one part was lost, and it is unknown which one. A part randomly extracted from the box (after transportation) turned out to be standard. Find the probability that the lost part was: a) a standard part; b) a non-standard part.
Solution. a) The extracted standard part obviously could not have been lost; any of the other 30 parts could have been lost $(21+10-1=30)$, and among them, 20 were standard $(21-1=20)$. The probability that a standard part was lost, $P=20 / 30=2 / 3$.

b) Among the 30 parts, each of which could have been lost, 10 were non-standard. The probability that a non-standard part was lost, $P=10 / 30=1 / 3$.