A box contains 150 oranges.If one orange is taken…

A box contains 150 oranges.If one orange is taken out from the box at random and the probability of its being rotten is 0.06, then find the number of good oranges in the box.

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Number of bad ones: $150\cdot 0.06=9$? –  Mårten W Jan 31 '13 at 16:06
What have you tried? –  Thomas Andrews Jan 31 '13 at 16:06
Strictly speaking, we don't know the number of good oranges. There could be 150 bad oranges in the box. We can only give the estimated number of good oranges. –  akkkk Jan 31 '13 at 16:07
@akkkk: We are told that if we take an orange at random from this box, the probability of bad is $0.06$. It follows that there are for sure exactly $9$ bad. –  André Nicolas Jan 31 '13 at 16:18
@MårtenW 0.06 is the probability of a rotten one. The question asks for the number of good ones, thus $150 \times (1 - 0.06) = 141$. –  Michael Jan 31 '13 at 17:01
The probability that the orange is not rotten (good) is $1 - 0.06 = 0.94$. Hence, we know that $0.94$ times the total number of oranges ($150$) is the number of good oranges, which numerically is $150 \times 0.94 =141$.
@nye The answer can be $91$ if and only if good does not mean not bad in your question. –  Parth Kohli Jan 31 '13 at 16:17