Let T1 and T2 denote the events that a randomly chosen bulb is of Type 1 and Type 2, respectively.
Since the shop has an equal number of bulbs of both types:
P(T1)=P(T2)=0.5 Let E be the event that a bulb lasts more than 100 hours. The given conditional probabilities are:
P(E∣T1)=0.7 P(E∣T2)=0.4 By the Law of Total Probability:
P(E)=P(E∣T1)P(T1)+P(E∣T2)P(T2) P(E)=(0.7)(0.5)+(0.4)(0.5)=0.35+0.20=0.55 Thus, the required probability is 0.55.