A certain light bulb manufacturer makes two types of bulbs, a low- cost short- life ( S-type) bulb and a more expensive long- life ( L- type) bulb. The two types of bulbs look identical and so the company must be sure to carefully label the boxes of bulbs. A box of bulbs is found on the floor of the manufacturing plant that (you guessed it) has not been labeled. In order to determine which types of bulbs are in the box, a promising young engineer suggested that they take one bulb from the box and run it until it burns out. After observing how long the bulb remains lit, they should be able to make a good guess as to which type of bulbs is in the box. It is known that the length of time (in hours), X, that a bulb lasts can be described by a geometric random variable PX (k) = (1 – a) ak, k = 0, 1, 2…
The parameter that appears in the above expression is for the S- type bulbs and for the L- type bulbs. It is known that of all the light bulbs the company manufactures 75% are S- type and 25% are L- type. Hence, before the experiment is run, the box in question has a 75% chance of being S- type and 25% chance of being L- type.
(a) If, after running the proposed experiment, it is observed that the bulb burned out after 200 hours, which type of bulb is most likely in the unmarked box? Mathematically justify your answer.
(b) What is the probability that your decision in part (a) turns out to be wrong? That is, if you decided that the box most likely contained L- type bulbs, what is the probability that the box actually contains S- type bulbs (or if you decided the box most likely contained S- type bulbs, what is the probability that the box actually contains L- type bulbs)?

  • CreatedNovember 19, 2015
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