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A lot consists of 144 ball pens of which 20 are defective and the others are good. Nuri will buy a pen if it is good, but will not buy if it is defective. The shopkeeper draws one pen at random and gives it to her. What is the probability that She wil

21.(i)    A lot consists of 144 ball pens of which 20 are defective and the others are good. Nuri will buy a pen if it is good, but will not buy if it is defective. The shopkeeper draws one pen at random and gives it to her. What is the probability that (i) She will buy it ?

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Total number of pens = 144

Total number of defective pens = 20

\therefore Number of good pens = 144-20 = 124

She will buy if the pen is good.

Therefore, the probability that she buys = probability that the pen is good = 

P(getting\ a\ good\ pen) = \frac{number\ of\ good\ pens}{total\ pens}= \frac{124}{144}

= \frac{31}{36}

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