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Probability question

2022 · Shift 2 · Q35
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  5. /2022 · Shift 2 · Q35

Probability question

2022 · Shift 2 · Q35

JEE AdvancedMathematicsProbabilityMCQ+3 / −1
Suppose that Box-I contains 8 red, 3 blue and 5 green balls, Box-II contains 24 red, 9 blue and 15 green balls, Box-III contains 1 blue, 12 green and 3 yellow balls, Box-IV contains 10 green, 16 orange and 6 white balls. A ball is chosen randomly from Box-I; call this ball bbb. If bbb is red then a ball is chosen randomly from Box-II, if bbb is blue then a ball is chosen randomly from Box-III, and if bbb is green then a ball is chosen randomly from Box-IV. The conditional probability of the event 'one of the chosen balls is white' given that the event 'at least one of the chosen balls is green' has happened, is equal to
  1. A
    15256\frac{15}{256}25615​
  2. B
    316\frac{3}{16}163​
  3. C
    552\frac{5}{52}525​
  4. D
    18\frac{1}{8}81​
View written solutionFree

Correct answer: C

Step-by-step Solution

1. Define the experimental setup and initial probabilities.

Let's denote the contents of the boxes and the probabilities of choosing a ball of a certain color from Box-I.

  • Box-I: 8 Red (R1), 3 Blue (B1), 5 Green (G1). Total = 16 balls.

    • P(R1)=816=12P(R_1) = \frac{8}{16} = \frac{1}{2}P(R1​)=168​=21​
    • P(B1)=316P(B_1) = \frac{3}{16}P(B1​)=163​
    • P(G1)=516P(G_1) = \frac{5}{16}P(G1​)=165​
  • Box-II (if R1 is chosen): 24 Red (R2), 9 Blue (B2), 15 Green (G2). Total = 48 balls.

  • Box-III (if B1 is chosen): 1 Blue (B3), 12 Green (G3), 3 Yellow (Y3). Total = 16 balls.

  • Box-IV (if G1 is chosen): 10 Green (G4), 16 Orange (O4), 6 White (W4). Total = 32 balls.

2. Define the events for conditional probability.

We need to find the conditional probability P(E∣F)P(E|F)P(E∣F), where:

  • EEE = The event that 'one of the chosen balls is white'.
  • FFF = The event that 'at least one of the chosen balls is green'.

The formula for conditional probability is P(E∣F)=P(E∩F)P(F)P(E|F) = \frac{P(E \cap F)}{P(F)}P(E∣F)=P(F)P(E∩F)​.

3. Calculate the probability of event F, P(F)P(F)P(F).

Event FFF ('at least one green ball') can happen in the following mutually exclusive ways:

  • Case 1: The first ball from Box-I is green (G1G_1G1​). The second ball can be anything. The condition is already met.

    • The probability of this case is P(G1)=516P(G_1) = \frac{5}{16}P(G1​)=165​.
  • Case 2: The first ball is red (R1R_1R1​) and the second ball is green (G2G_2G2​).

    • P(R1∩G2)=P(R1)×P(G2∣R1)=816×1548=12×516=532P(R_1 \cap G_2) = P(R_1) \times P(G_2 | R_1) = \frac{8}{16} \times \frac{15}{48} = \frac{1}{2} \times \frac{5}{16} = \frac{5}{32}P(R1​∩G2​)=P(R1​)×P(G2​∣R1​)=168​×4815​=21​×165​=325​.
  • Case 3: The first ball is blue (B1B_1B1​) and the second ball is green (G3G_3G3​).

    • P(B1∩G3)=P(B1)×P(G3∣B1)=316×1216=316×34=964P(B_1 \cap G_3) = P(B_1) \times P(G_3 | B_1) = \frac{3}{16} \times \frac{12}{16} = \frac{3}{16} \times \frac{3}{4} = \frac{9}{64}P(B1​∩G3​)=P(B1​)×P(G3​∣B1​)=163​×1612​=163​×43​=649​.

The total probability of event FFF is the sum of the probabilities of these cases: P(F)=P(G1)+P(R1∩G2)+P(B1∩G3)P(F) = P(G_1) + P(R_1 \cap G_2) + P(B_1 \cap G_3)P(F)=P(G1​)+P(R1​∩G2​)+P(B1​∩G3​) P(F)=516+532+964P(F) = \frac{5}{16} + \frac{5}{32} + \frac{9}{64}P(F)=165​+325​+649​ To add these fractions, we find a common denominator, which is 64: P(F)=5×416×4+5×232×2+964=2064+1064+964=3964P(F) = \frac{5 \times 4}{16 \times 4} + \frac{5 \times 2}{32 \times 2} + \frac{9}{64} = \frac{20}{64} + \frac{10}{64} + \frac{9}{64} = \frac{39}{64}P(F)=16×45×4​+32×25×2​+649​=6420​+6410​+649​=6439​

4. Calculate the probability of the intersection event, P(E∩F)P(E \cap F)P(E∩F).

  • Event EEE ('one white ball') can only occur if the second ball is white. White balls are only in Box-IV. To choose from Box-IV, the first ball must be green (G1G_1G1​).
  • So, event EEE is the sequence of drawing a green ball from Box-I and then a white ball from Box-IV. P(E)=P(G1∩W4)=P(G1)×P(W4∣G1)=516×632=516×316=15256P(E) = P(G_1 \cap W_4) = P(G_1) \times P(W_4 | G_1) = \frac{5}{16} \times \frac{6}{32} = \frac{5}{16} \times \frac{3}{16} = \frac{15}{256}P(E)=P(G1​∩W4​)=P(G1​)×P(W4​∣G1​)=165​×326​=165​×163​=25615​
  • Now consider the intersection event E∩FE \cap FE∩F: 'one white ball' AND 'at least one green ball'.
  • If event EEE occurs, the first ball drawn is green (G1G_1G1​). This means the condition for event FFF ('at least one green ball') is automatically satisfied.
  • Therefore, event EEE is a subset of event FFF (E⊂FE \subset FE⊂F), which implies that E∩F=EE \cap F = EE∩F=E.
  • So, P(E∩F)=P(E)=15256P(E \cap F) = P(E) = \frac{15}{256}P(E∩F)=P(E)=25615​.

5. Calculate the conditional probability P(E∣F)P(E|F)P(E∣F).

Using the formula for conditional probability: P(E∣F)=P(E∩F)P(F)=15/25639/64P(E|F) = \frac{P(E \cap F)}{P(F)} = \frac{15/256}{39/64}P(E∣F)=P(F)P(E∩F)​=39/6415/256​ P(E∣F)=15256×6439P(E|F) = \frac{15}{256} \times \frac{64}{39}P(E∣F)=25615​×3964​ Since 256=4×64256 = 4 \times 64256=4×64, we can simplify: P(E∣F)=154×39P(E|F) = \frac{15}{4 \times 39}P(E∣F)=4×3915​ Now, we can simplify the fraction by dividing the numerator and denominator by 3: P(E∣F)=3×54×(3×13)=54×13=552P(E|F) = \frac{3 \times 5}{4 \times (3 \times 13)} = \frac{5}{4 \times 13} = \frac{5}{52}P(E∣F)=4×(3×13)3×5​=4×135​=525​

Thus, the required conditional probability is 552\frac{5}{52}525​.

Conclusion: Comparing the result with the given options, the correct option is C.

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