# Discrete Math HELP!?

Suppose that you buy a lottery ticket containing k distinct numbers from among

{1, 2, . . . , n} where 1 ≤ k ≤ n. To determine the winning tickets, k balls are randomly drawn without replacement from a bin containing n balls numbered 1, 2, . . . , n.

(a) What is the probability that at least one of the numbers on your lottery ticket is among those drawn from the bin?(b) Prizes start when you have at least (k /2) (ie. half) numbers in common with the drawn balls. Express the probability that you win something. You may find it helpful to use summation notation.

*I did question 1 but I am unaware on how to do question 2. I don't get what the question is asking me. Can someone please help me out? Thanks.

### 1 Answer

- AlanLv 71 month ago
(b) part b uses the hyper-geometric probability distribution

let j = number of matches

P(j > = K/2 ) = P( j=floor( ((k/2) + 0.5) ) + P( j= floor ((k/2) + 1.5) )

+ P( j= floor((k/2) + 2.5) + ... + P(j= (k-1) ) + P(j= k)

To find each probability you can treat that as hyper-geometric probability

So in general ,pick exactly k items in subset of size n chosen

from an original Group size N where there were K of the

particular item in the larger group

This cases can be solved with the hypergeometric

distribution

https://en.wikipedia.org/wiki/Hypergeometric_distr...

P(X =k) = ( ( K over k )( (N-K) over (n-k)) / ( N over n )

===where "over" means combination

or in terms of combinations

P(X=k) = (C(K,k)*C( (N-K), n-k)/ ( C(N,n)

But in this case what does N,n,K, k

equal

N = n

n = k

K = k

k = j and

j increments as indicated above

so putting in the formula

P(X = j ) = ( C(k, j) * C( n-k, k-j) / (C(n,k)

P(X = j) =C(k,j) * C(n-k, k -j ) / C(n,k)

P( you win ) = from j = floor ( (k/2) + 0.5) to k

Σ C(k,j)* C(n--k,k-j)/C(n,k)

Example N = 20

k = 6 f

loor( 6/2 + 1/2) = 3

P( you win ) = from 3 to 6 Σ C(6,j)*C(14,6-j)/ C (20,6)