91Ó°ÊÓ

A box contains a large number of transistors, 30 per cent of which ate type \(A\) and the rest type \(B\). A random sample of 4 transistors is taken. Determine the probabilities that they are: (a) all of type \(A\) (b) all of type \(B\) (c) two of type \(A\) and two of type \(B\) (d) three of type \(A\) and one of type \(B\).

A large stock of resistors is known to have 20 per cent defectives. If 5 resistors are drawn at random, determine: (a) the probabilities that (i) none is defective (ii) at least two are defective (b) the mean and standard deviation of the distribution of defects.

Light bulbs, having a mean life of 2400 hours and standard deviation of 62 hours, are used for a consignment of 4000 bulbs. Determine: (a) the number of bulhs likely to have a life in excess of 2500 hours. (b) the percentage of bulbs with a life length between 2300 hours and 2500 hours (c) the probability of any one bulb having a life of 2500 hours (to the nearest hour).