Metal casting processes are notoriously slow and expensive A

Metal casting processes are notoriously slow and expensive. A common problem facing many older casting processes is flashing. In casting, liquid metal is shot into a mold and rapidly cooled. A flash commonly forms on the piece at the spot in the mold where the metal flows. Sanding corrects minor problems with flashing, but severe problems may require scrapping the part. A particular casting operation historically has scrapped 10% of its parts due to severe problems with flashing.

c. Find the expected number of parts until the first one is scrapped, the variance, and the standard deviation.

e. Find the expected number of parts until the fourth part is scrapped, the variance, and the standard deviation.

These are the correct answers, but I\'m not sure how to get them:

c. 10; 90; 9.48

e. 40; 360; 18.97

Solution