madeeasytest

Question

a coin tossed nine time .what is the probability that only first two tosses will yield tails? 

Comments

$\frac{1}{512}$

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it will look like, $TTHHH....H$ and hence probability = $\frac{1}{2^9}$

Answer 1

A fair coin is tossed 9 times,what is the probability that tail will come up in first two toss of coin"

We do not even need to consider remaining tosses.

So Total Set of outcomes of First 2 tosses is = ${ HH,HT,TH,TT}$

Required outcome = ${TT}$

Answer -> cardinality ${TT}$ / cardinality ${HH,HT,TH,TT} = 1/4 = 0.25$

Comments

@Aditya Tewari,

without seeing entire conversation, you posted that link, i guess.

a coin tossed nine time .what is the probability that only first two tosses will yield tails? 

see the highlighted word here and compare it with one whose link you have posted. 

the word 'only' is the cause of different answer.

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@joshi...did read the thread btw

yes yes...i see now.....now i get you clearly...thanks for pointing out...:)

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If you for 9 tosses also answer will be the same.

If you tossed 9 times then the out come possible are : 2^9

We have to find the possibilities by putting first and second toss as Tail so : TT------- (2^7 possibilities are there). First 2 tosses result is TAIL and remaining tosses output can be anything so 2^7 possibilities).

 

P(first 2 tosses Tail)=possibilities of TT-------/total no. of possibilities

=(2^7)/(2^9)

=1/(2^2)

=1/4

=0.25