# Me and Probabilitys are having a problem

• Jan 31st 2007, 03:47 PM
air2slb
Me and Probabilitys are having a problem
Ive got a few questions if someone could help out. Thanks so much. I love this site.

1. In one town of 54% democrates of 2 randoml y selected for a survey. Find the probability that they are both democrates??

2. Find the probability of correctly answering the 1st 2 questions on a multiple choice test if random questions are made and each question have 5 possible answers?

3. You are dealt 2 cards from a shuffled deck of 52 playing cards. find the probability that both cards will be black.

If you could show the break down so i can get the consept.. Thank you. I wish i could help someone. Not in this subject.
• Jan 31st 2007, 10:32 PM
CaptainBlack
Quote:

Originally Posted by air2slb
Ive got a few questions if someone could help out. Thanks so much. I love this site.

1. In one town of 54% democrates of 2 randoml y selected for a survey. Find the probability that they are both democrates??

Probability that the first selected individual is a democrat is 0.54 (which is
54% written as a percentage). Probability that the second is a democrat is
also 0.54.

So the probability that they are both democrats is 0.54*0.54=0.2196, or

RonL
• Jan 31st 2007, 10:36 PM
CaptainBlack
Quote:

Originally Posted by air2slb
2. Find the probability of correctly answering the 1st 2 questions on a multiple choice test if random questions are made and each question have 5 possible answers?

The probability of getting the right answer to a question by choosing answers
randomly is 1/5=0.2 (or 20%), as there ia one correct answer out of a total
of five possibilities.

Now the answer proceeds as for question 1.

Probability of getting first question right is 0.2, and of getting the second
right is also 0.2. So the probability of getting both right is 0.2*0.2=0.04
(or 4%).

Note that this works because the result of the first choice does not effect
the second choice. We will see in the next question a situation where this
is not true.

RonL
• Jan 31st 2007, 10:40 PM
CaptainBlack
Quote:

Originally Posted by air2slb
3. You are dealt 2 cards from a shuffled deck of 52 playing cards. find the probability that both cards will be black.

The probability that the first card selected is black is 26/52, as there are
26 black cards amoung a total of 52 cards.

Now the probability that the second card is black given that the first was
black is 25/51, as there are 25 black cards remaining out of a total or 51 cards remaining.

So the probability that both cards are black is:

(26/52)*(25/51) ~= .2451,

RonL
• Feb 1st 2007, 03:36 PM
air2slb
The answer on the test was shown as 25/102. I know that comes out to be 0.245 but how would I have gotten the 25/102?
• Feb 1st 2007, 09:20 PM
CaptainBlack
Quote:

Originally Posted by air2slb
The answer on the test was shown as 25/102. I know that comes out to be 0.245 but how would I have gotten the 25/102?

(26/52)(25/51)=(1/2)(25/51)=25/(51*2)=25/102

RonL