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5 coins are tossed. If two of them show heads, then the probability that all 5 coins show head is

a. 1/32 b. 1/10 c. 1/26 d. 1/13 e. none of the above

Total outcome:2^5=32
Outcome of at least two of them show heads:
Total-(1 head)-(0 head)=32-5-1=26
Outcome of 5 heads=1
Probability=1/26
_________________

Keep on asking, and it will be given you;
keep on seeking, and you will find;
keep on knocking, and it will be opened to you.

Last edited by HongHu on 01 Apr 2005, 10:50, edited 1 time in total.

Sorry, the 1 is for 0 head, and the 5 is for 1 head. I'll edit the post to make it clearer.

Basically the question implies that two of them are heads but the other three can also be heads. So we need to know what probability is for getting at least two heads. It'll be total minus the cases where we only get one head, and where we get no heads.
_________________

Keep on asking, and it will be given you;
keep on seeking, and you will find;
keep on knocking, and it will be opened to you.

There are two explanations for this sentence "two of the coins show heads". One is that exactly two coins are heads. The other is at least two are heads. The next sentence askes "What probability is all coins show heads". You know that means in additional to the two heads, the other coins may show heads too. In other word there would be two or more heads, or at least two heads.
_________________

Keep on asking, and it will be given you;
keep on seeking, and you will find;
keep on knocking, and it will be opened to you.

Sorry guys but I don't get this one. the question is :

5 coins are tossed. If two of them show heads, then the probability that all 5 coins show head is ?

We already know that 2 of them showed heads, no ? In my opnion it should be 1/8 becasue there are still 3 toss to make and the events are independant : (1/2)^3