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Probability Question

"An unbiased uniform random number generator, which generates numbers between 0 and 1 inclusive, is used to generate two numbers A and B. Find the probability that A will be greater than B."

I can see four possibilities:

A B
0 0
0 1
1 0
1 1


Only 1 of them, A > B. So why is the correct answer 1/2?. The explanation I got was, P(A>B) = P(B>A) => P(A>B) = 1/2. But that doesn't account for A=B?

EDIT: Just realised the inclusive bit. But can someone explain to me the logic of P(A>B) = P(B>A) => P(A>B) = 1/2? The generator can still generate 2 of the same numbers?
(edited 10 years ago)
I think the question means the number generator gives random numbers that aren't necessarily whole numbers
Reply 2
Original post by cant_think_of_name
I think the question means the number generator gives random numbers that aren't necessarily whole numbers


Yep, I reread the question a second after I posted and realised my misunderstanding, my confusion still stands though. What happens when A=B?
Original post by dada55
Yep, I reread the question a second after I posted and realised my misunderstanding, my confusion still stands though. What happens when A=B?


Well what's the probabilty that A=B?
Reply 4
Original post by ghostwalker
Well what's the probabilty that A=B?


hmm, if there are an infinite amount of random numbers between 0 and 1 then the probability is 0? That would then make sense I guess.
Reply 5
Original post by dada55
EDIT: Just realised the inclusive bit. But can someone explain to me the logic of P(A>B) = P(B>A) => P(A>B) = 1/2? The generator can still generate 2 of the same numbers?


Not sure if you get this now, but think about P(A<B) + P(A>B) = 1. Since P(A<B) and P(A>B) absorb all possibilities, we know the sum of these must = 1. Rearrange to get 1 - P(A<B) = P(A>B). We have P(A>B) = P(B>A), transforming the equation to 1 - P(A<B) = P(A<B), which becomes 1 = 2P(A<B) and from this you get the result!
Reply 6
Original post by Jooooshy
Not sure if you get this now, but think about P(A<B) + P(A>B) = 1. Since P(A<B) and P(A>B) absorb all possibilities, we know the sum of these must = 1. Rearrange to get 1 - P(A<B) = P(A>B). We have P(A>B) = P(B>A), transforming the equation to 1 - P(A<B) = P(A<B), which becomes 1 = 2P(A<B) and from this you get the result!


Thanks for that, that makes sense. I'm right then that if the question said that only integers between 0 and 1 inclusive can be generated. Then P(A>B) = P(B>A) = 1/4 and P(A=B) = 1/2 ?
Reply 7
Original post by dada55
Thanks for that, that makes sense. I'm right then that if the question said that only integers between 0 and 1 inclusive can be generated. Then P(A>B) = P(B>A) = 1/4 and P(A=B) = 1/2 ?


Yes :-)

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