# stats

Watch
#1
1.Fair four sided dice with faces labelled 1,2,3 and 4 are rolled one at a time until a 4 is scored
given that a four is not rolled in the first six attempts , find the probability that a four is rolled within the next two attempts.
I tried (3/4)^6×((3/4)+(3/4x1/4))

2. Carrie is playing a game of Solitaire. The probability that she wins each game is 0.2.Carrie plays the game until she wins
find the variance of the number of games Carrie plays. This is (1-p)/p^2 which is 0.8/0.2^2 =20 but i don't really understand why could someone please explain why this is how you calculate the variance in this case.
0
1 month ago
#2
(Original post by learningizk00l)
1.Fair four sided dice with faces labelled 1,2,3 and 4 are rolled one at a time until a 4 is scored
given that a four is not rolled in the first six attempts , find the probability that a four is rolled within the next two attempts.
I tried (3/4)^6×((3/4)+(3/4x1/4))

2. Carrie is playing a game of Solitaire. The probability that she wins each game is 0.2.Carrie plays the game until she wins
find the variance of the number of games Carrie plays. This is (1-p)/p^2 which is 0.8/0.2^2 =20 but i don't really understand why could someone please explain why this is how you calculate the variance in this case.

This makes sense as OR , and may be used because the rolls are independent, but it really seems to me that you should be looking for , which is what you have done.

In terms of your second question, I can't tell you the reasons why, but think about your modelling. She's playing a game where the probability of success is constant, and each game is independent. She's also playing until she has a single success. This means it makes sense to model this as a Geometric Distribution with probability 0.2.

It therefore comes that the variance of this is as this is how we calculate the variance of such a distribution
1
X

new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### Poll

Join the discussion

#### What factors affect your mental health the most right now?

4.84%
Uncertainty around my education (158)
15.61%
Uncertainty around my future career prospects (113)
11.17%
Lack of purpose or motivation (146)
14.43%
Lack of support system (eg. teachers, counsellors, delays in care) (42)
4.15%
Impact of lockdown on physical health (56)
5.53%
Loneliness (84)
8.3%
Financial worries (34)
3.36%
Concern about myself or my loves ones getting/having been ill (48)
4.74%
Exposure to negative news/social media (50)
4.94%
Lack of real life entertainment (54)
5.34%
Lack of confidence in making big life decisions (89)
8.79%
Worry about missed opportunities during the pandemic (89)
8.79%