DSE 2010 Duck, Amit Sir, please help. urgent

[Rosy]

Hey could someone tell me how to solve Q 4 from dse 2010? I know its easy, just cant seem to get the trick.  here's the link. http://economicsentrance.weebly.com/uploads/1/1/0/5/1105777/option-a-2010.pdf

[Rosy]

I mean if the number of tosses required to get a head for the first time is 3, then should the probability be 1/8? the probability of getting TTH

[Pooja]

http://discussion-forum.2150183.n2.nabble.com/dse-2010-q4-td6651872.html#a6652488

[Rosy]

Aah. Thanks MR :) i wasnt able to find it on the group.

[Prerna Rakheja]

Hi Rosy,

Here X ~ Geometric(1/2)

Where probability of head (success) = 1/2 = p, thus q = 1 - p = 1/2
Since X is defined as the number of tosses in the question thus number of failures is 2 (as tosses = 3 where head is obtained in the last toss)

Pr(X <= k) = ∑〖q^kp〗Where k = 0, 1, 2 (Value of the distribution function at X = k in case of geometric)

Just compute the above and you will get your answer :)