# Thread: Independent Trials & Discrete Random Variables

1. ## Independent Trials & Discrete Random Variables

I have a few questions, wondering if anyone can help me here:

Q. All batteries made by Power Corp. have only probability 0.20 of delivering the advertised voltage. What is the smallest possible number of these batteries that Jack should buy in order to be at least 60% sure of having at least one battery that delivers the advertised voltage?
- I approached this problem by first finding the probability that buying one battery will give him the advertised voltage, 20% = 1/5
I know if he buys 5 batteries then he will have a 100% chance that he gets one with advertised voltage. I know that 60% of 5 = 5 x 6/10 = 30/10 = 3.
Therefore my answer is 3. --> This question seemed to simple to me so I think I may be wrong (or understood the question wrong), I don't really know what other approach to use for it (without using a calculator)?

Q. A 3-card hand is dealt from a standard deck of cards. The discrete random variable X counts the number of clubs in the hand. Considering the sample space to be the set of all possible 3-card hands which could be dealt, so that n(s) = (52 choose 3), how many sample points are in the event (X = 1)?
- When X = 1, according to the problem (if I understand correctly) then 1/3 cards in the hand is a club and the other 2 are not clubs.
If (39 choose 3) x (13 choose 0) is no club being drawn, then (13 choose 1) x (39 choose 2) is the probability of 1 club being drawn?
(13 choose 1) x (39 choose 2)

Q. How many possible values does the random variable B(25, .3) have?
- Not enough information to determine? I think B(x,X) means that when x = 25 one value of Pr[X] = .3? I could be totally off.

2. This is not a homework service
So just what do you expect us to do?
This is a help service. We cannot help if you make no effort to work the problems.

Moreover, posting a list of problems is in keeping with the spirit of this free site.

3. Originally Posted by Plato
This is not a homework service
So just what do you expect us to do?
This is a help service. We cannot help if you make no effort to work the problems.

Moreover, posting a list of problems is in keeping with the spirit of this free site.
Understandable, I'll edit the post and show what I have done so far then. I just wanted to see if my answers matched anyone elses.