Probability Qs random sample + w/standard deviation

[brunettegurl]

Hi

1) Suppose that it is known that in a certain large population,10%of is is colourblind. If a random sample of 25 people is drawn from the population, find the probability that exactly 8 of them are colourblind.

My Take: is to use the Poisson Probability: f(x)= (e^^-$$\lambda$$)* $$\lambda$$ ^x/x! where $$\lambda$$= 0.1 and do it for x=1,2,3...till 8
Im not sure if my take is correct.

2) Supposse that the cholesterol values for a certain population are approx. normally w/mean=200 and standard deviation 20. 90% of the population have cholesterol values greater than x. Find x

My Take: 0.90= P(X$$\leq$$x)
0.90= P(X-200/20$$\leq$$x-200/20)
0.90=P(z$$\leq$$x-200/20)

z_0.90=1.20
x= 20*z_0.90+ 200
=224

the answer should be 174.4

any help would be appreciated

Thanks

 

[mathman]

For question 1, binomial would be exact. [25!/(8!17!)].1⁸.9¹⁷

For question 2, the question was "greater than". Your answer is for "less than".