A box of bolts contain 8 thick bolt, and 3 thin bolts. A box of nuts contains 6 that fit the thick bolts, 4 that fit the medium bolts, and 2 that fit the thin bolts. One bolt and nut are nut are chosen at random. What is the probability that the nut fits the bolt?
Solution
The sample space consists of all pairs of nuts and bolts, and each pair is equally likely to be chosen. The event that the nut fits the bolt corresponds to the set of all matching pairs of nuts and bolts. Therefore
P(nut fits bolt) =(number of maiching pairs of nuts and bolts)/(number of pairs of nuts and bolts)
There are 6 + 4 + 2 = 12 nuts, and 8 + 5 + 3 = 16 bolts. Therefore
Number of pairs of nuts and bolts = (12)(16) = 192
The number of matching pairs is found by summing the number of pairs of thick nuts and bolts, the number of pairs of medium nuts and bolts, and the number of pairs of thin nuts and bolts, These number are
Number of pairs of thick nuts and bolts = (6)(8) = 48
Number of pairs of medium nuts and bolts = (4)(5) = 20
Number of pairs of thin nuts and bolts = (2)(3) = 6
Therefore
P(nuts fits bolts) = (48+20+6)/192 = 0.3854