GMAT Prep2? Head and Tails

[dferm]

Almost got it but not quite there..

Can someone put some light into this question..

Thanks..

[VP_Jim]

Probability questions with the word "exactly" are tough. I think the easiest way to do these is just to list out the different possibilities:

HHH
HHT
HTH
HTT
TTT
TTH
THT
THH

Now we just need to find the ones that have exactly two "H"s in them, or 3/8 = .375.

Alternatively, you can draw a probability diagram (see attached file). This illustrates all of the possibilities that you might have. Then, just follow the branches of the diagram to find the ones that have exactly two "H"s.

The drawing is pretty crude, but it gets the point across!

[senthil]

It involves two findings .
Probability of getting two heads and one tail
no of ways of getting it

HHT , no of ways u can get this is 3!/2! = 3 ways
Equal chances of head and tail therefore the probability of this combination will be (1/2*1/2*1/2)=1/8.
therefore , the no of ways of one can get ...... = 3/8 =.375