You are Here: Home >< Maths

S2 Jan 2011 Q7 - Probability watch

1. Hi,

I don't understand how the answer to part d has been worked out (question and model answers attached).

Would really appreciate an explanation.
Attached Images

2. (Original post by TheRandomGenius)
Hi,

I don't understand how the answer to part d has been worked out (question and model answers attached).

Would really appreciate an explanation.
In a Binomial distribution, P(X=r) = nCr(p)^r(1-p)^1-r

So for at least 2, you have the probability from part (c), so you can sub in the equation above; Also nCr is just the number of combinations, but that's easy to work out as 3, if there were 3 people a, b and c, the combinations of two people queuing for more than 5 minutes are a,b or a,c or b,c. But then you have to add on the probability of all 3 queuing for more than 5 minutes, as it says at least 2 in the question.
3. (Original post by NotNotBatman)
In a Binomial distribution, P(X=r) = nCr(p)^r(1-p)^1-r

So for at least 2, you have the probability from part (c), so you can sub in the equation above; Also nCr is just the number of combinations, but that's easy to work out as 3, if there were 3 people a, b and c, the combinations of two people queuing for more than 5 minutes are a,b or a,c or b,c. But then you have to add on the probability of all 3 queuing for more than 5 minutes, as it says at least 2 in the question.
Ahhh they used the binomial formula... Can't believe I missed that😭 shameful...

Thanks!

Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 9, 2017
Today on TSR

Summer Bucket List is Back!

Start yours and you could win £150!

Poll
Useful resources

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

How to use LaTex

Writing equations the easy way

Study habits of A* students

Top tips from students who have already aced their exams

Chat with other maths applicants