# Simple probability -

• Mar 19th 2010, 09:21 AM
WannaBe
Simple probability -
Hey there everyone.. Here is a question I will be delighted to get some verification on:

A man is trying to shoot a bullet into a marksmanship squared board.
In every shooting attemp, the probability that he won't hit the board at all is 0.2 .
If he hits the board, the probability that he will hit an area that will give him points is 0.9.
If he hits an area that gives him points, the probability that he will hit an area that will give 10 points is 0.2 , 20 points is 0,3 and 30 points is 0.5.

A. What is the probability that in a random shooting attemp, the shooter will get 10 points?

B. Let's assume that in a specific shooting attempt, the shooter didn't get points.
What is the probability that the reason for this is that he hit the board but in an area that doesn't give points?

I'll be delighted to get some help in part B...The answer to part A is 0.144 (Hope I did it right...) ...How shold I solve part B? What is the condition in it?

Thanks!
• Mar 19th 2010, 10:14 AM
u2_wa
Quote:

Originally Posted by WannaBe
Hey there everyone.. Here is a question I will be delighted to get some verification on:

A man is trying to shoot a bullet into a marksmanship squared board.
In every shooting attemp, the probability that he won't hit the board at all is 0.2 .
If he hits the board, the probability that he will hit an area that will give him points is 0.9.
If he hits an area that gives him points, the probability that he will hit an area that will give 10 points is 0.2 , 20 points is 0,3 and 30 points is 0.5.

A. What is the probability that in a random shooting attemp, the shooter will get 10 points?

B. Let's assume that in a specific shooting attempt, the shooter didn't get points.
What is the probability that the reason for this is that he hit the board but in an area that doesn't give points?

I'll be delighted to get some help in part B...The answer to part A is 0.144 (Hope I did it right...) ...How shold I solve part B? What is the condition in it?

Thanks!

Hello WannaBe:

b) The shooter did not get points, it means:

P(He did not hit the target) $\displaystyle = 0.2$

P[He hit the board]$\displaystyle =\color{blue}0.8$
P(He hit but did not get points) $\displaystyle =0.8*0.1=0.08$

P(He did not get points)$\displaystyle =0.2+0.8*0.1=0.28$

P(he hit the board but in an area that did not get
points)$\displaystyle =\frac{0.08}{0.28}=\frac{2}{7}$

Hope this helps
• Mar 20th 2010, 01:11 AM
WannaBe
Actually I think you're wrong... We know that:
P(He did not hit the target) = 0.2 indeed, but:
P(He hit but did not get points) = 0.8*0.1=0.08...
Hence:P(He did not get points) = 0.28
and then: P(he hit the board but in an area that did not get
points) = 0.08/0.28=2/7

:) Well, thanks a lot anyway :) You've verified my calculation :)
• Mar 20th 2010, 04:22 AM
u2_wa
Oh I made a blunder(Angry). Sorry for that!

The main strategy to solve was correct!!!
• Mar 20th 2010, 06:38 AM
WannaBe
NVM man, as I said in my previous msg - your strategy gave me the verification I needed and I thank you for this...