MCQMediumJEE 2023Probability Basics

JEE Mathematics 2023 Question with Solution

Fifteen football players of a club are given 1515 T-shirts with their names written on the back. If the players pick up the T-shirts randomly, then the probability that at least 33 players pick the correct T-shirt is:

  • A

    524\frac{5}{24}

  • B

    215\frac{2}{15}

  • C

    16\frac{1}{6}

  • D

    536\frac{5}{36}

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: There are 1515 players and 1515 named T-shirts, and each player picks one T-shirt at random.

Find: The probability that at least 33 players pick the correct T-shirt.

The solution states that the correct option is A. It also presents the setup

P=1D15+15C1D14+15C2D1315!P = 1 - \frac{D_{15} + {}^{15}C_1 \cdot D_{14} + {}^{15}C_2 \cdot D_{13}}{15!}

which corresponds to subtracting the cases with exactly 00, 11, or 22 correct picks.

Using the derangement notation shown in the solution,

D15=15!e,D14=14!e,D13=13!eD_{15} = \frac{15!}{e}, \quad D_{14} = \frac{14!}{e}, \quad D_{13} = \frac{13!}{e}

so the displayed expression becomes

P=115!e+15C114!e+15C213!e15!P = 1 - \frac{\frac{15!}{e} + {}^{15}C_1 \cdot \frac{14!}{e} + {}^{15}C_2 \cdot \frac{13!}{e}}{15!}

the solution then simplifies this approximately and reaches a numerical discussion, but its written intermediate approximation is inconsistent with its own final marked option.

Since the solution explicitly declares Option A as correct, the answer is taken as A, which is

524\frac{5}{24}

Therefore, the correct option is A.

Discrepancy Noted from the solution

Given: The source the solution marks Option A as correct.

Find: The final answer to be recorded.

The solution contains a contradiction:

  1. It explicitly says The Correct Option is A.
  2. In the concluding text, it says the required probability is approximately 16\frac{1}{6}, which corresponds to Option C.
  3. The numerical line shown there, 0.920.92, is also inconsistent with 16\frac{1}{6}.

Because the page explicitly labels the correct option as A, and the worked approximation is internally inconsistent, the safest grounded extraction is to record the answer as A and preserve the discrepancy in the solution notes.

Therefore, the extracted correct answer is 524\frac{5}{24}, i.e. Option A.

Common mistakes

  • Students may count permutations with at least 33 correct picks directly. That becomes cumbersome and error-prone. Instead, count the complement: exactly 00, 11, and 22 correct picks, then subtract from 11.

  • A common mistake is to use ordinary permutations instead of derangements for the remaining players. After fixing some correct picks, the others must all be incorrect, so derangement counts such as D14D_{14} or D13D_{13} are required.

  • Students may trust a contradictory final numeric line without checking consistency. Here the solution itself contains inconsistent approximation statements, so the explicitly marked correct option must be used carefully.

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