Answer:
a) P=0.1721
b) P=0.3528
c) P=0.3981
Step-by-step explanation:
This sampling can be modeled by a binominal distribution where p is the probability of a project to belong to the first section and q the probability of belonging to the second section.
a) In this case we have a sample size of n=15.
The value of p is p=25/(25+35)=0.4167 and q=1-0.4167=0.5833.
The probability of having exactly 10 projects for the second section is equal to having exactly 5 projects of the first section.
This probability can be calculated as:
![P=\frac{n!}{(n-k)!k!}p^kq^{n-k}= \frac{15!}{(10)!5!}\cdot 0.4167^5\cdot0.5833^{10}=0.1721](https://tex.z-dn.net/?f=P%3D%5Cfrac%7Bn%21%7D%7B%28n-k%29%21k%21%7Dp%5Ekq%5E%7Bn-k%7D%3D%20%5Cfrac%7B15%21%7D%7B%2810%29%215%21%7D%5Ccdot%200.4167%5E5%5Ccdot0.5833%5E%7B10%7D%3D0.1721)
b) To have at least 10 projects from the 2nd section, means we have at most 5 projects for the first section. In this case, we have to calculate the probability for k=0 (every project belongs to the 2nd section), k=1, k=2, k=3, k=4 and k=5.
We apply the same formula but as a sum:
![P(k\leq5)=\sum_{k=0}^{5}\frac{n!}{(n-k)!k!}p^kq^{n-k}](https://tex.z-dn.net/?f=P%28k%5Cleq5%29%3D%5Csum_%7Bk%3D0%7D%5E%7B5%7D%5Cfrac%7Bn%21%7D%7B%28n-k%29%21k%21%7Dp%5Ekq%5E%7Bn-k%7D)
Then we have:
![P(k=0)=0.0003\\P(k=1)=0.0033\\P(k=2)=0.0165\\P(k=3)=0.0511\\P(k=4)=0.1095\\P(k=5)=0.1721\\\\P(k\leq5)=0.0003+0.0033+0.0165+0.0511+0.1095+0.1721=0.3528](https://tex.z-dn.net/?f=P%28k%3D0%29%3D0.0003%5C%5CP%28k%3D1%29%3D0.0033%5C%5CP%28k%3D2%29%3D0.0165%5C%5CP%28k%3D3%29%3D0.0511%5C%5CP%28k%3D4%29%3D0.1095%5C%5CP%28k%3D5%29%3D0.1721%5C%5C%5C%5CP%28k%5Cleq5%29%3D0.0003%2B0.0033%2B0.0165%2B0.0511%2B0.1095%2B0.1721%3D0.3528)
c) In this case, we have the sum of the probability that k is equal or less than 5, and the probability tha k is 10 or more (10 or more projects belonging to the 1st section).
The first (k less or equal to 5) is already calculated.
We have to calculate for k equal to 10 or more.
![P(k\geq10)=\sum_{k=10}^{15}\frac{n!}{(n-k)!k!}p^kq^{n-k}](https://tex.z-dn.net/?f=P%28k%5Cgeq10%29%3D%5Csum_%7Bk%3D10%7D%5E%7B15%7D%5Cfrac%7Bn%21%7D%7B%28n-k%29%21k%21%7Dp%5Ekq%5E%7Bn-k%7D)
Then we have
![P(k=10)=0.0320\\P(k=11)=0.0104\\P(k=12)=0.0025\\P(k=13)=0.0004\\P(k=14)=0.0000\\P(k=15)=0.0000\\\\P(k\geq10)=0.032+0.0104+0.0025+0.0004+0+0=0.0453](https://tex.z-dn.net/?f=P%28k%3D10%29%3D0.0320%5C%5CP%28k%3D11%29%3D0.0104%5C%5CP%28k%3D12%29%3D0.0025%5C%5CP%28k%3D13%29%3D0.0004%5C%5CP%28k%3D14%29%3D0.0000%5C%5CP%28k%3D15%29%3D0.0000%5C%5C%5C%5CP%28k%5Cgeq10%29%3D0.032%2B0.0104%2B0.0025%2B0.0004%2B0%2B0%3D0.0453)
The sum of the probabilities is
![P(k\leq5)+P(k\geq10)=0.3528+0.0453=0.3981](https://tex.z-dn.net/?f=P%28k%5Cleq5%29%2BP%28k%5Cgeq10%29%3D0.3528%2B0.0453%3D0.3981)