Probability - ISI Qsn!

2 candidates contested for an election, A & B. A got a votes, while B got b. Given a>b, find probability that A was ahead of B - throughout the competition..... SHOULD IT NOT BE JUST 12??????????????

11 Answers

62
Lokesh Verma ·

no Soumik I don't think it should be 1/2

simply because it should depend on a and b.

like take the case wehn B got zero votes. the probability is 1!

11
virang1 Jhaveri ·

Is it a/a+b

62
Lokesh Verma ·

I think the best way to solve it would be through a axb grid...

What do you guys think?

341
Hari Shankar ·

I think that and the concept of Catalan Numbers does the trick. we seem to need to ignore the number of ways climbing the grid without ever going above the line y = x and that smacks of Catalan numbers.

62
Lokesh Verma ·

One thing that is most commendable about prophet sir is that he knows the names of every trick under the sun :D

I din know that this was somethign related to Catalan number or infact it existed even though i have used this trick several times !!

341
Hari Shankar ·

i am not very familiar with it, but it did remind of that concept. Lets see

11
Devil ·

virang, ans is given as a-ba+b

62
Lokesh Verma ·

http://en.wikipedia.org/wiki/Bertrand%27s_ballot_theorem

Read this one..

Even i read catalan numbers last month..

I thought that this could be pulled off by recursion but now it seems unlikely....

see the proof on the page.. and the solution

btw I wont be here for 2 days.. bye till then [1]

3
msp ·

the no of votes that A has more than B=a-b

so the probability that A can lead the election =a-b/a+b.

i hope this is rong,due to the above discussion,i just want to know why i am rong.

3
msp ·

this thread left uncared.ne1 help me.

62
Lokesh Verma ·

I dont get the logic sankara.. I mean how is it obvious to you.. to me it is not at all obvious!

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