Math Is Fun Forum

simone

Member

Registered: 2007-10-05

Posts: 19

help please, confused about probability!

HELP!!!

could you please help on this question?

If a coin has an equal probability of landing heads up or tails up each time it is flipped, what is the probability that the coin will land heads up exactly twice in 3 consecutive flips?

I calculated   (1/2)(1/2)(1/2)=1/8    , since the probability of getting heads twice in two tosses is 1/4, and then I'd have one more toss which is another independent event, so   (1/4)(1/2).

What am I doing wrong? I'm worried I'm making a major mistake!

pi man

Member

Registered: 2006-07-06

Posts: 251

Re: help please, confused about probability!

You would be right if the question was what are the chances of flipping 2 heads followed by a tails.   What about THH and HTH?   They also have a 1/8 chance of happening so add these together to get 3/8.

And when the problems involve a relatively limited set of possibilities, it's often easy to list them all to verify/reject your solution.
HHH
HHT   good
HTH   good
THH   good
HTT
THT
TTH
TTT

simone

Member

Registered: 2007-10-05

Posts: 19

Re: help please, confused about probability!

thank you so much.:P
could you take a look at the other problem I posted too?

Last edited by simone (2007-10-14 11:42:39)