Binomial Halloween
1. **Problem statement:** We have a binomial random variable $X$ representing the number of Canadians (out of 20) who will spend money on Halloween. The probability of success (spending money) is $p=0.63$.
2. **Part (a):** Compute $P(X=14)$.
The binomial probability formula is:
$$P(X=k) = \binom{n}{k} p^k (1-p)^{n-k}$$
where $n=20$, $k=14$, $p=0.63$.
Calculate:
$$P(X=14) = \binom{20}{14} (0.63)^{14} (0.37)^6$$
Using a calculator or software:
$$\binom{20}{14} = 38760$$
$$0.63^{14} \approx 0.0067$$
$$0.37^{6} \approx 0.0022$$
So,
$$P(X=14) \approx 38760 \times 0.0067 \times 0.0022 = 0.5721$$
3. **Part (b):** Compute $P(8 \leq X \leq 14)$.
This is the sum of probabilities from $k=8$ to $k=14$:
$$P(8 \leq X \leq 14) = \sum_{k=8}^{14} \binom{20}{k} (0.63)^k (0.37)^{20-k}$$
Using binomial cumulative distribution function (CDF) or software, we find:
$$P(8 \leq X \leq 14) \approx 0.8283$$
4. **Part (c):** Compute expected value $E(X)$ and standard deviation $SD(X)$.
For a binomial distribution:
$$E(X) = np = 20 \times 0.63 = 12.60$$
$$SD(X) = \sqrt{np(1-p)} = \sqrt{20 \times 0.63 \times 0.37} = \sqrt{4.662} \approx 2.16$$
5. **Part (d):** Compute the probability that the 10-th Canadian chosen is the 6-th to say they will spend money.
This is a negative binomial probability:
$$P(Y=10) = \binom{10-1}{6-1} p^6 (1-p)^{10-6} = \binom{9}{5} (0.63)^6 (0.37)^4$$
Calculate:
$$\binom{9}{5} = 126$$
$$0.63^6 \approx 0.0635$$
$$0.37^4 \approx 0.0187$$
So,
$$P(Y=10) = 126 \times 0.0635 \times 0.0187 \approx 0.1497$$
**Final answers:**
- $P(X=14) = 0.5721$
- $P(8 \leq X \leq 14) = 0.8283$
- $E(X) = 12.60$
- $SD(X) = 2.16$
- $P(\text{10-th is 6-th success}) = 0.1497$