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Probability question

2022 · Shift 1 · Q34
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Probability question

2022 · Shift 1 · Q34

JEE AdvancedMathematicsProbabilityMCQ+3 / −1

Two players, P1P_{1}P1​ and P2P_{2}P2​, play a game against each other. In every round of the game, each player rolls a fair die once, where the six faces of the die have six distinct numbers. Let xxx and yyy denote the readings on the die rolled by P1P_{1}P1​ and P2P_{2}P2​, respectively. If x>yx>yx>y, then P1P_{1}P1​ scores 5 points and P2P_{2}P2​ scores 0 point. If x=yx=yx=y, then each player scores 2 points. If x<yx \lt yx<y, then P1P_{1}P1​ scores 0 point and P2P_{2}P2​ scores 5 points. Let XiX_{i}Xi​ and YiY_{i}Yi​ be the total scores of P1P_{1}P1​ and P2P_{2}P2​, respectively, after playing the ith i^{\text {th }}ith  round.

List-I List-II
(I) Probability of (X2≥Y2)\left(X_{2} \geq Y_{2}\right)(X2​≥Y2​) is (P) 38\frac{3}{8}83​
(II) Probability of (X2>Y2)\left(X_{2}>Y_{2}\right)(X2​>Y2​) is (Q) 1116\frac{11}{16}1611​
(III) Probability of (X3=Y3)\left(X_{3}=Y_{3}\right)(X3​=Y3​) is (R) 516\frac{5}{16}165​
(IV) Probability of (X3>Y3)\left(X_{3}>Y_{3}\right)(X3​>Y3​) is (S) 355864\frac{355}{864}864355​
(T) 77432\frac{77}{432}43277​

The correct option is:

  1. A
    (I) →\rightarrow→(Q); (II) →\rightarrow→(R); (III) →\rightarrow→(T); (IV) →(S)\rightarrow(S)→(S)
  2. B
    (I) →\rightarrow→(Q); (II) →\rightarrow→(R); (III) →\rightarrow→(T); (IV) →\rightarrow→ (T)
  3. C
    (I) →\rightarrow→(P); (II) →\rightarrow→(R); (III) →(Q);(IV)→(S)\rightarrow(\mathrm{Q}) ;(\mathrm{IV}) \rightarrow(\mathrm{S})→(Q);(IV)→(S)
  4. D
    (I) →\rightarrow→(P); (II) →\rightarrow→(R); (III) →\rightarrow→(Q); (IV) →\rightarrow→ (T)
View written solutionFree

Correct answer: A

Step 1: Analyze the probabilities of outcomes in a single round.

Let xxx be the reading on the die rolled by player P1P_1P1​ and yyy be the reading on the die rolled by player P2P_2P2​. Both are fair six-sided dice. The total number of possible outcomes in a single round is 6×6=366 \times 6 = 366×6=36.

The scoring rules lead to three possible events in a round:

  1. P1P_1P1​ wins (W): This happens if x>yx > yx>y. The number of favorable outcomes is calculated by summing the possibilities for each value of yyy:

    • If y=1y=1y=1, x∈{2,3,4,5,6}x \in \{2,3,4,5,6\}x∈{2,3,4,5,6} (5 outcomes)
    • If y=2y=2y=2, x∈{3,4,5,6}x \in \{3,4,5,6\}x∈{3,4,5,6} (4 outcomes)
    • If y=3y=3y=3, x∈{4,5,6}x \in \{4,5,6\}x∈{4,5,6} (3 outcomes)
    • If y=4y=4y=4, x∈{5,6}x \in \{5,6\}x∈{5,6} (2 outcomes)
    • If y=5y=5y=5, x∈{6}x \in \{6\}x∈{6} (1 outcome) Total outcomes for x>yx>yx>y is 5+4+3+2+1=155+4+3+2+1=155+4+3+2+1=15. The probability is p1=P(W)=1536=512p_1 = P(W) = \frac{15}{36} = \frac{5}{12}p1​=P(W)=3615​=125​. Scores: P1P_1P1​ gets 5 points, P2P_2P2​ gets 0 points.
  2. Draw (D): This happens if x=yx = yx=y. The favorable outcomes are (1,1),(2,2),(3,3),(4,4),(5,5),(6,6)(1,1), (2,2), (3,3), (4,4), (5,5), (6,6)(1,1),(2,2),(3,3),(4,4),(5,5),(6,6). There are 6 such outcomes. The probability is pd=P(D)=636=16=212p_d = P(D) = \frac{6}{36} = \frac{1}{6} = \frac{2}{12}pd​=P(D)=366​=61​=122​. Scores: Both players get 2 points.

  3. P2P_2P2​ wins (L, for P1P_1P1​ loses): This happens if x<yx < yx<y. By symmetry with the x>yx>yx>y case, there are 15 favorable outcomes. The probability is p2=P(L)=1536=512p_2 = P(L) = \frac{15}{36} = \frac{5}{12}p2​=P(L)=3615​=125​. Scores: P1P_1P1​ gets 0 points, P2P_2P2​ gets 5 points.

Check: p1+pd+p2=512+212+512=1212=1p_1 + p_d + p_2 = \frac{5}{12} + \frac{2}{12} + \frac{5}{12} = \frac{12}{12} = 1p1​+pd​+p2​=125​+122​+125​=1212​=1.

Step 2: Calculate probabilities for 2 rounds.

Let the outcomes of the two rounds be represented by a sequence of two letters (e.g., WD means P1P_1P1​ wins round 1, and round 2 is a draw). X2X_2X2​ and Y2Y_2Y2​ are the total scores after 2 rounds.

  • WW: X2=5+5=10,Y2=0+0=0  ⟹  X2>Y2X_2=5+5=10, Y_2=0+0=0 \implies X_2 > Y_2X2​=5+5=10,Y2​=0+0=0⟹X2​>Y2​. Prob: p12=(512)2=25144p_1^2 = (\frac{5}{12})^2 = \frac{25}{144}p12​=(125​)2=14425​.
  • WD, DW: X2=5+2=7,Y2=0+2=2  ⟹  X2>Y2X_2=5+2=7, Y_2=0+2=2 \implies X_2 > Y_2X2​=5+2=7,Y2​=0+2=2⟹X2​>Y2​. Prob: 2p1pd=2(512)(212)=201442 p_1 p_d = 2(\frac{5}{12})(\frac{2}{12}) = \frac{20}{144}2p1​pd​=2(125​)(122​)=14420​.
  • WL, LW: X2=5+0=5,Y2=0+5=5  ⟹  X2=Y2X_2=5+0=5, Y_2=0+5=5 \implies X_2 = Y_2X2​=5+0=5,Y2​=0+5=5⟹X2​=Y2​. Prob: 2p1p2=2(512)(512)=501442 p_1 p_2 = 2(\frac{5}{12})(\frac{5}{12}) = \frac{50}{144}2p1​p2​=2(125​)(125​)=14450​.
  • DD: X2=2+2=4,Y2=2+2=4  ⟹  X2=Y2X_2=2+2=4, Y_2=2+2=4 \implies X_2 = Y_2X2​=2+2=4,Y2​=2+2=4⟹X2​=Y2​. Prob: pd2=(212)2=4144p_d^2 = (\frac{2}{12})^2 = \frac{4}{144}pd2​=(122​)2=1444​.
  • DL, LD: X2=2+0=2,Y2=2+5=7  ⟹  X2<Y2X_2=2+0=2, Y_2=2+5=7 \implies X_2 < Y_2X2​=2+0=2,Y2​=2+5=7⟹X2​<Y2​. Prob: 2pdp2=2(212)(512)=201442 p_d p_2 = 2(\frac{2}{12})(\frac{5}{12}) = \frac{20}{144}2pd​p2​=2(122​)(125​)=14420​.
  • LL: X2=0+0=0,Y2=5+5=10  ⟹  X2<Y2X_2=0+0=0, Y_2=5+5=10 \implies X_2 < Y_2X2​=0+0=0,Y2​=5+5=10⟹X2​<Y2​. Prob: p22=(512)2=25144p_2^2 = (\frac{5}{12})^2 = \frac{25}{144}p22​=(125​)2=14425​.

(II) Probability of (X2>Y2)(X_2 > Y_2)(X2​>Y2​): This corresponds to the outcomes WW, WD, DW. P(X2>Y2)=P(WW)+P(WD)+P(DW)=25144+20144=45144P(X_2 > Y_2) = P(WW) + P(WD) + P(DW) = \frac{25}{144} + \frac{20}{144} = \frac{45}{144}P(X2​>Y2​)=P(WW)+P(WD)+P(DW)=14425​+14420​=14445​. Simplifying the fraction: 45144=9×59×16=516\frac{45}{144} = \frac{9 \times 5}{9 \times 16} = \frac{5}{16}14445​=9×169×5​=165​. This matches List-II option (R). So, (II) →\rightarrow→ (R).

(I) Probability of (X2≥Y2)(X_2 \geq Y_2)(X2​≥Y2​): This is P(X2>Y2)+P(X2=Y2)P(X_2 > Y_2) + P(X_2 = Y_2)P(X2​>Y2​)+P(X2​=Y2​). P(X2=Y2)P(X_2 = Y_2)P(X2​=Y2​) corresponds to WL, LW, DD. P(X2=Y2)=P(WL)+P(LW)+P(DD)=50144+4144=54144P(X_2 = Y_2) = P(WL) + P(LW) + P(DD) = \frac{50}{144} + \frac{4}{144} = \frac{54}{144}P(X2​=Y2​)=P(WL)+P(LW)+P(DD)=14450​+1444​=14454​. P(X2≥Y2)=45144+54144=99144P(X_2 \geq Y_2) = \frac{45}{144} + \frac{54}{144} = \frac{99}{144}P(X2​≥Y2​)=14445​+14454​=14499​. Simplifying the fraction: 99144=9×119×16=1116\frac{99}{144} = \frac{9 \times 11}{9 \times 16} = \frac{11}{16}14499​=9×169×11​=1611​. This matches List-II option (Q). So, (I) →\rightarrow→ (Q).

Step 3: Calculate probabilities for 3 rounds.

Let NW,ND,NLN_W, N_D, N_LNW​,ND​,NL​ be the number of wins for P1P_1P1​, draws, and losses for P1P_1P1​ in 3 rounds, respectively. We have NW+ND+NL=3N_W+N_D+N_L=3NW​+ND​+NL​=3. The total scores are X3=5NW+2NDX_3 = 5N_W + 2N_DX3​=5NW​+2ND​ and Y3=5NL+2NDY_3 = 5N_L + 2N_DY3​=5NL​+2ND​.

(III) Probability of (X3=Y3)(X_3 = Y_3)(X3​=Y3​): The condition X3=Y3X_3 = Y_3X3​=Y3​ implies 5NW+2ND=5NL+2ND5N_W + 2N_D = 5N_L + 2N_D5NW​+2ND​=5NL​+2ND​, which simplifies to NW=NLN_W = N_LNW​=NL​. Possible combinations for (NW,ND,NL)(N_W, N_D, N_L)(NW​,ND​,NL​) satisfying NW+ND+NL=3N_W+N_D+N_L=3NW​+ND​+NL​=3 and NW=NLN_W = N_LNW​=NL​:

  1. (NW,ND,NL)=(0,3,0)(N_W, N_D, N_L) = (0, 3, 0)(NW​,ND​,NL​)=(0,3,0): All three rounds are draws (DDD). The number of such sequences is 1. Probability is pd3=(212)3=81728p_d^3 = (\frac{2}{12})^3 = \frac{8}{1728}pd3​=(122​)3=17288​.
  2. (NW,ND,NL)=(1,1,1)(N_W, N_D, N_L) = (1, 1, 1)(NW​,ND​,NL​)=(1,1,1): One win, one draw, one loss. The number of sequences (permutations of WDL) is 3!=63! = 63!=6. Probability for one sequence is p1pdp2=512212512=501728p_1 p_d p_2 = \frac{5}{12} \frac{2}{12} \frac{5}{12} = \frac{50}{1728}p1​pd​p2​=125​122​125​=172850​. Total probability for this case is 6×501728=30017286 \times \frac{50}{1728} = \frac{300}{1728}6×172850​=1728300​. Total probability: P(X3=Y3)=81728+3001728=3081728P(X_3 = Y_3) = \frac{8}{1728} + \frac{300}{1728} = \frac{308}{1728}P(X3​=Y3​)=17288​+1728300​=1728308​. Simplifying: 3081728=154864=77432\frac{308}{1728} = \frac{154}{864} = \frac{77}{432}1728308​=864154​=43277​. This matches List-II option (T). So, (III) →\rightarrow→ (T).

(IV) Probability of (X3>Y3)(X_3 > Y_3)(X3​>Y3​): The condition X3>Y3X_3 > Y_3X3​>Y3​ implies 5NW+2ND>5NL+2ND5N_W + 2N_D > 5N_L + 2N_D5NW​+2ND​>5NL​+2ND​, which simplifies to NW>NLN_W > N_LNW​>NL​. We sum the probabilities for all combinations (NW,ND,NL)(N_W, N_D, N_L)(NW​,ND​,NL​) that satisfy this condition:

  1. (3,0,0)(3, 0, 0)(3,0,0): (WWW) (33)p13=1⋅(512)3=1251728\binom{3}{3}p_1^3 = 1 \cdot (\frac{5}{12})^3 = \frac{125}{1728}(33​)p13​=1⋅(125​)3=1728125​.
  2. (2,1,0)(2, 1, 0)(2,1,0): (WWD) (32,1,0)p12pd=3⋅(512)2(212)=1501728\binom{3}{2,1,0}p_1^2 p_d = 3 \cdot (\frac{5}{12})^2(\frac{2}{12}) = \frac{150}{1728}(2,1,03​)p12​pd​=3⋅(125​)2(122​)=1728150​.
  3. (2,0,1)(2, 0, 1)(2,0,1): (WWL) (32,0,1)p12p2=3⋅(512)2(512)=3751728\binom{3}{2,0,1}p_1^2 p_2 = 3 \cdot (\frac{5}{12})^2(\frac{5}{12}) = \frac{375}{1728}(2,0,13​)p12​p2​=3⋅(125​)2(125​)=1728375​.
  4. (1,2,0)(1, 2, 0)(1,2,0): (WDD) (31,2,0)p1pd2=3⋅(512)(212)2=601728\binom{3}{1,2,0}p_1 p_d^2 = 3 \cdot (\frac{5}{12})(\frac{2}{12})^2 = \frac{60}{1728}(1,2,03​)p1​pd2​=3⋅(125​)(122​)2=172860​. Total probability: P(X3>Y3)=125+150+375+601728=7101728P(X_3 > Y_3) = \frac{125 + 150 + 375 + 60}{1728} = \frac{710}{1728}P(X3​>Y3​)=1728125+150+375+60​=1728710​. Simplifying: 7101728=355864\frac{710}{1728} = \frac{355}{864}1728710​=864355​. This matches List-II option (S). So, (IV) →\rightarrow→ (S).

Step 4: Conclusion

The matching is as follows:

  • (I) →\rightarrow→ (Q)
  • (II) →\rightarrow→ (R)
  • (III) →\rightarrow→ (T)
  • (IV) →\rightarrow→ (S)

This corresponds to option A.

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