The Student Room Group

Another binomial distribution q

b) A man steps out of a bar into an alley that runs East to West. A each step, he chooses at random whether to go East or West, after 12 steps he takes a rest.

i) Why is it impossible for him to be one step away from the bar?
ii) What is the probability he is then 10 steps East of the bar?

For the ii) q, I thought it'd be 12C10 x (1/2)^10 x (1/2)^2 which looks correct to me seeing as P(East) = 6/12 and P(West) = 6/12 and I'm finding the prob of going 10 one way and 2 the other.. I get the probability being 0.016, the textbook says 0.00293.

How do I get that answer??
Original post by Magenta96

How do I get that answer??


10 steps East and 2 steps West, does not put you 10 steps East of the bar.

Try again.
Original post by Magenta96
b) A man steps out of a bar into an alley that runs East to West. A each step, he chooses at random whether to go East or West, after 12 steps he takes a rest.
i) Why is it impossible for him to be one step away from the bar?
ii) What is the probability he is then 10 steps East of the bar?
For the ii) q, I thought it'd be 12C10 x (1/2)^10 x (1/2)^2 which looks correct to me seeing as P(East) = 6/12 and P(West) = 6/12 and I'm finding the prob of going 10 one way and 2 the other.. I get the probability being 0.016, the textbook says 0.00293.
How do I get that answer??

It should be 11 steps east and 1 step west.

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