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Probability question

2023 · 15 Apr · Shift 1 · Q32
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  5. /2023 · 15 Apr · Shift 1 · Q32

Probability question

2023 · 15 Apr · Shift 1 · Q32

JEE MainMathematicsProbabilityMCQ+4 / −1
A bag contains 6 white and 4 black balls. A die is rolled once and the number of balls equal to the number obtained on the die are drawn from the bag at random. The probability that all the balls drawn are white is :
  1. A
    14\frac{1}{4}41​
  2. B
    950\frac{9}{50}509​
  3. C
    15\frac{1}{5}51​
  4. D
    1150\frac{11}{50}5011​
View written solutionFree

Correct answer: C

  1. Let the number obtained on the die be kkk.

    Since a fair die is rolled once, each of k=1,2,3,4,5,6k=1,2,3,4,5,6k=1,2,3,4,5,6 occurs with probability 16\frac{1}{6}61​.

  2. If kkk balls are drawn, the probability that all kkk are white is (6k)(10k)\frac{\binom{6}{k}}{\binom{10}{k}}(k10​)(k6​)​ because there are 666 white balls out of total 101010 balls.

  3. Hence the required probability is P=∑k=16P(die=k)⋅P(all white∣k)P=\sum_{k=1}^{6} P(\text{die}=k)\cdot P(\text{all white}\mid k)P=∑k=16​P(die=k)⋅P(all white∣k) =16∑k=16(6k)(10k).=\frac{1}{6}\sum_{k=1}^{6}\frac{\binom{6}{k}}{\binom{10}{k}}.=61​∑k=16​(k10​)(k6​)​.

  4. Compute each term:

    • For k=1k=1k=1: (61)(101)=610=35\frac{\binom{6}{1}}{\binom{10}{1}}=\frac{6}{10}=\frac{3}{5}(110​)(16​)​=106​=53​

    • For k=2k=2k=2: (62)(102)=1545=13\frac{\binom{6}{2}}{\binom{10}{2}}=\frac{15}{45}=\frac{1}{3}(210​)(26​)​=4515​=31​

    • For k=3k=3k=3: (63)(103)=20120=16\frac{\binom{6}{3}}{\binom{10}{3}}=\frac{20}{120}=\frac{1}{6}(310​)(36​)​=12020​=61​

    • For k=4k=4k=4: (64)(104)=15210=114\frac{\binom{6}{4}}{\binom{10}{4}}=\frac{15}{210}=\frac{1}{14}(410​)(46​)​=21015​=141​

    • For k=5k=5k=5: (65)(105)=6252=142\frac{\binom{6}{5}}{\binom{10}{5}}=\frac{6}{252}=\frac{1}{42}(510​)(56​)​=2526​=421​

    • For k=6k=6k=6: (66)(106)=1210\frac{\binom{6}{6}}{\binom{10}{6}}=\frac{1}{210}(610​)(66​)​=2101​

  5. Add them: 35+13+16+114+142+1210.\frac{3}{5}+\frac{1}{3}+\frac{1}{6}+\frac{1}{14}+\frac{1}{42}+\frac{1}{210}.53​+31​+61​+141​+421​+2101​.

    Taking LCM 210210210, 126+70+35+15+5+1210=252210=65.\frac{126+70+35+15+5+1}{210}=\frac{252}{210}=\frac{6}{5}.210126+70+35+15+5+1​=210252​=56​.

    Therefore, P=16⋅65=15.P=\frac{1}{6}\cdot \frac{6}{5}=\frac{1}{5}.P=61​⋅56​=51​.

  6. So the correct option is: C 15\boxed{\text{C } \frac{1}{5}}C 51​​

  7. Checking options systematically:

    • A: 14\frac{1}{4}41​ — incorrect
    • B: 950\frac{9}{50}509​ — incorrect
    • C: 15\frac{1}{5}51​ — correct
    • D: 1150\frac{11}{50}5011​ — incorrect
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