A bag contains 3 red balls and nn yellow balls, where n≥2n \ge 2. In a game, Joe removes 2 balls at random from the bag one at a time without replacement. The number of red balls Joe removes from the bag is denoted by TT.

  1. (i)

    Find P(T=t)\mathrm{P}(T = t) for all possible values of tt.

  2. (ii)

    Find E(T)\mathrm{E}(T) and Var(T)\mathrm{Var}(T).

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Worked solution

10(i)

1. Identify the total number of balls and the possible values of $t$.

The total number of balls in the bag initially is n+3n + 3.
Since Joe draws 2 balls and there are at least 2 red balls and 2 yellow balls (as n≥2n \ge 2), the number of red balls drawn, tt, can be 0,1,0, 1, or 22.

2. Calculate the probability of drawing 0 red balls (i.e. 2 yellow balls).

P(T=0)=P(Yellow, Yellow)\mathrm{P}(T=0) = \mathrm{P}(\text{Yellow, Yellow})
=nn+3×n−1n+2= \frac{n}{n+3} \times \frac{n-1}{n+2}
=n(n−1)(n+3)(n+2)= \frac{n(n-1)}{(n+3)(n+2)}

3. Calculate the probability of drawing exactly 1 red ball.

P(T=1)=P(Red, Yellow)+P(Yellow, Red)\mathrm{P}(T=1) = \mathrm{P}(\text{Red, Yellow}) + \mathrm{P}(\text{Yellow, Red})
=(3n+3×nn+2)+(nn+3×3n+2)= \left(\frac{3}{n+3} \times \frac{n}{n+2}\right) + \left(\frac{n}{n+3} \times \frac{3}{n+2}\right)
=3n(n+3)(n+2)+3n(n+3)(n+2)= \frac{3n}{(n+3)(n+2)} + \frac{3n}{(n+3)(n+2)}
=6n(n+3)(n+2)= \frac{6n}{(n+3)(n+2)}

4. Calculate the probability of drawing 2 red balls.

P(T=2)=P(Red, Red)\mathrm{P}(T=2) = \mathrm{P}(\text{Red, Red})
=3n+3×2n+2= \frac{3}{n+3} \times \frac{2}{n+2}
=6(n+3)(n+2)= \frac{6}{(n+3)(n+2)}

Answer

P(T=0)=n(n−1)(n+3)(n+2)\mathrm{P}(T=0) = \frac{n(n-1)}{(n+3)(n+2)}
P(T=1)=6n(n+3)(n+2)\mathrm{P}(T=1) = \frac{6n}{(n+3)(n+2)}
P(T=2)=6(n+3)(n+2)\mathrm{P}(T=2) = \frac{6}{(n+3)(n+2)}

10(ii)

1. Determine the required probabilities from part (i)

From part (i), the probabilities of removing 1 or 2 red balls are:
P(T=1)=2×3n+3×nn+2=6n(n+3)(n+2)\mathrm{P}(T=1) = 2 \times \frac{3}{n+3} \times \frac{n}{n+2} = \frac{6n}{(n+3)(n+2)}
P(T=2)=3n+3×2n+2=6(n+3)(n+2)\mathrm{P}(T=2) = \frac{3}{n+3} \times \frac{2}{n+2} = \frac{6}{(n+3)(n+2)}

2. Calculate the expected value $\mathrm{E}(T)$

E(T)=∑t⋅P(T=t)\mathrm{E}(T) = \sum t \cdot \mathrm{P}(T=t)
=0+1⋅6n(n+3)(n+2)+2⋅6(n+3)(n+2)= 0 + 1 \cdot \frac{6n}{(n+3)(n+2)} + 2 \cdot \frac{6}{(n+3)(n+2)}
=6n+12(n+3)(n+2)= \frac{6n + 12}{(n+3)(n+2)}
=6(n+2)(n+3)(n+2)=6n+3= \frac{6(n+2)}{(n+3)(n+2)} = \frac{6}{n+3}

3. Calculate $\mathrm{E}(T^2)$ to prepare for the variance calculation

E(T2)=∑t2⋅P(T=t)\mathrm{E}(T^2) = \sum t^2 \cdot \mathrm{P}(T=t)
=0+12⋅6n(n+3)(n+2)+22⋅6(n+3)(n+2)= 0 + 1^2 \cdot \frac{6n}{(n+3)(n+2)} + 2^2 \cdot \frac{6}{(n+3)(n+2)}
=6n+24(n+3)(n+2)= \frac{6n + 24}{(n+3)(n+2)}
=6(n+4)(n+3)(n+2)= \frac{6(n+4)}{(n+3)(n+2)}

4. Calculate the variance $\mathrm{Var}(T)$ and simplify the algebraic fraction

Var(T)=E(T2)−[E(T)]2\mathrm{Var}(T) = \mathrm{E}(T^2) - [\mathrm{E}(T)]^2
=6(n+4)(n+3)(n+2)−(6n+3)2= \frac{6(n+4)}{(n+3)(n+2)} - \left( \frac{6}{n+3} \right)^2
=6(n+4)(n+3)−36(n+2)(n+3)2(n+2)= \frac{6(n+4)(n+3) - 36(n+2)}{(n+3)^2(n+2)}
=6(n2+7n+12)−36n−72(n+3)2(n+2)= \frac{6(n^2 + 7n + 12) - 36n - 72}{(n+3)^2(n+2)}
=6n2+42n+72−36n−72(n+3)2(n+2)= \frac{6n^2 + 42n + 72 - 36n - 72}{(n+3)^2(n+2)}
=6n2+6n(n+3)2(n+2)= \frac{6n^2 + 6n}{(n+3)^2(n+2)}
=6n(n+1)(n+3)2(n+2)= \frac{6n(n+1)}{(n+3)^2(n+2)}

Answer

E(T)=6n+3\mathrm{E}(T) = \frac{6}{n+3}, Var(T)=6n(n+1)(n+3)2(n+2)\mathrm{Var}(T) = \frac{6n(n+1)}{(n+3)^2(n+2)}

AI analysis — may contain mistakes.

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