A bag contains 3 red balls and yellow balls, where . 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 .
- (i)
Find for all possible values of .
- (ii)
Find and .
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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 .
Since Joe draws 2 balls and there are at least 2 red balls and 2 yellow balls (as ), the number of red balls drawn, , can be or .
2. Calculate the probability of drawing 0 red balls (i.e. 2 yellow balls).
3. Calculate the probability of drawing exactly 1 red ball.
4. Calculate the probability of drawing 2 red balls.
10(ii)
1. Determine the required probabilities from part (i)
From part (i), the probabilities of removing 1 or 2 red balls are:
2. Calculate the expected value $\mathrm{E}(T)$
3. Calculate $\mathrm{E}(T^2)$ to prepare for the variance calculation
4. Calculate the variance $\mathrm{Var}(T)$ and simplify the algebraic fraction
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