Thread: Probability?

Right, so I've got an upcoming exam tomorrow, and I just found out I'm stuck on probabilities (Not sure if that is what it's called in English).

Anyways, the question goes like this :

Troms skiing facility got 6 menn for NM in skiing. 2 of them are under the age of 20. It's a 4x7,5km lap, and they are going to pick 4 "runners(/skiiers)".

a) What's the probablity for that NONE of the 2 under the age of 20 does this?
b) What's the probablity for that minimum ONE of the 2 under the age of 20 does this?
c) What's the probablity for that EXACTLY ONE of the 2 under the age of 20 does this?
d)What's the probability that the skiier hit at least 4 of 5 targets with 5 shots.? (Hitting 1 target = 90%)

If you could explain your answers, that would be great.
I will most likely understand it, if you just set it up correctly.

(Not even sure if I've been trying the right formula :
X=x (n) p^x (1-p)^n-x
----(x) (<- not n divided by x)


OR if you could do :

Statistic over football matches in 2005 showed the probabilities for home victiory (H), tie (T), away victory (A).
P(H) = 0,45, P(U) = 0,29, P(B)= 0,26

a) What's the probability that ALL the twelve matches ends with home victory?
b) How big is the posibility that the 6 first ends with home victory, and the next tree tie, and the last tree with away victory?
c) Probability that there won't be an away victory?

Ok, on "c) Probability that there won't be an away victory?" I figured :
P(H) = 0,45 + P(U) = 0,29 divided by 1.

Does that add up?

Also, I'm not sure how to approach these questions, could anyone give any kind of hints?

Or a good explained answer?

Originally Posted by Rawr

Right, so I've got an upcoming exam tomorrow, and I just found out I'm stuck on probabilities (Not sure if that is what it's called in English).

Anyways, the question goes like this :

Troms skiing facility got 6 menn for NM in skiing. 2 of them are under the age of 20. It's a 4x7,5km lap, and they are going to pick 4 "runners(/skiiers)".

a) What's the probablity for that NONE of the 2 under the age of 20 does this?
b) What's the probablity for that minimum ONE of the 2 under the age of 20 does this?
c) What's the probablity for that EXACTLY ONE of the 2 under the age of 20 does this?
d)What's the probability that the skiier hit at least 4 of 5 targets with 5 shots.? (Hitting 1 target = 90%)

If you could explain your answers, that would be great.
I will most likely understand it, if you just set it up correctly.

[snip]

a) $\displaystyle \frac{{2 \choose 0} \cdot {4 \choose 2}}{{6 \choose 2}}$.


b) $\displaystyle \frac{{2 \choose 1} \cdot {4 \choose 1}}{{6 \choose 2}} + \frac{{2 \choose 2} \cdot {4 \choose 0}}{{6 \choose 2}} $.


c) $\displaystyle \frac{{2 \choose 1} \cdot {4 \choose 1}}{{6 \choose 2}}$.


d) X ~ Binomial(n = 5, p = 0.9). Calculate $\displaystyle \Pr(X \geq 4)$.

Originally Posted by Rawr

[snip]
Statistic over football matches in 2005 showed the probabilities for home victiory (H), tie (T), away victory (A).
P(H) = 0,45, P(U) = 0,29, P(B)= 0,26 Mr F says: What events do U and B represent?

a) What's the probability that ALL the twelve matches ends with home victory?
b) How big is the posibility that the 6 first ends with home victory, and the next tree tie, and the last tree with away victory?
c) Probability that there won't be an away victory?

a) (0,45)^12.

Thank you mr. fantastic, even though it was a day late.

I've already had my exam now .


I did however find out how to do it in time.
The formula I posted worked just fine, I think I accidentally wrote the wrong number or something when I tried it out.