A small company makes wine glasses. Each day, n randomly chosen wine glasses are checked and the number of wine glasses found to be cracked is denoted by X. Assume now that X has the distribution , where .
- (a)
State, in context of the question, two assumptions needed for X to be well modelled by a binomial distribution.
- (b)
Given that the mean of X and the variance of X are 1.8 and 1.773 respectively, find the value of n and the value of p.
- (c)
Given instead that the probability of finding 2 cracked wine glasses is thrice the probability of finding 3 cracked wine glasses, find p in terms of n.
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Worked solution
8(a)
1. State the independence assumption in the context of the question
Whether a randomly chosen wine glass is cracked is independent of whether any other chosen wine glass is cracked.
2. State the constant probability assumption in the context of the question
The probability that a randomly chosen wine glass is cracked is constant for all wine glasses.
- Whether any chosen wine glass is cracked is independent of whether any other chosen wine glass is cracked.
- The probability that a chosen wine glass is cracked is constant.
8(b)
1. Set up equations for the mean and variance
For :
2. Solve for $p$ by substituting the mean into the variance equation
3. Solve for $n$ using the value of $p$
,
8(c)
1. Set up the equation using the binomial probability formula.
2. Expand the combinations and simplify the equation.
Since and assuming , we can divide both sides by :
3. Solve for $p$ in terms of $n$.
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