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Currently Playing: The Probability Question Everyone Solves Wrong | JEE Advanced Level

The Probability Question Everyone Solves Wrong | JEE Advanced Level In today’s Factorial’s Question of the Day, we discuss a very interesting probability trap. A lot contains 20 articles. The probability that the lot contains exactly 2 defective articles is 0.4 and the probability that the lot contains exactly 3 defective articles is 0.6. Articles are drawn randomly one by one without replacement and are tested until all defective articles are found. Find the probability that the testing procedure ends at the twelfth testing. Most students immediately compute the answer as 99/1900. But that solution quietly assumes that the tester already knows how many defectives are present, which is never stated in the question. If the tester does not know the number of defectives, the procedure cannot stop after finding two defectives because a third one may still exist. Hence only the 3 defective case contributes, leading to the correct probability: Answer = 11/380 This problem beautifully highlights how interpreting the experiment correctly in probability is more important than blindly applying formulas. Join our Telegram for more such problems: https://t.me/academyfactorial #JEEAdvanced #JEEMaths #JEE2026 #JEEMains2026 #JEEMaths2026 #Probability #FactorialAcademy #JEEAdvancedMaths