pls help ! maths probability

P(Get 2 heads out of 3 times)
= P(HHT) + P(HTH) + P(THH)
= (1/2 x 1/2 x 1/2) + (1/2 x 1/2 x 1/2) + (1/2 x 1/2 x 1/2)
= 3(1/2 x 1/2 x 1/2)
= 3/8 (or 37.5%)
The probaaility of getting exactly 2 Heads is 3/8.

你可畫 tree diagram 黎塭



／H (H, H, H)
／H
＼T (H, H, T)
H
＼ ／H (H, T, H)
T
＼T (H, T, T)


／H (T, H, H)
／H
＼T (T, H, T)
T
＼ ／H (T, T, H)
T
＼T (T, T, T)

∴ The probability of getting exactly 2 heads is 3/8.

You can list all cases
Let H : Head ; T : Tail
Possible Combinations
HHH HHT HTH HTT TTT TTH THT THH
So the probability of getting exactly 2 Heads
=3/8

2008-12-23 19:45:26 補充：
http://img103.imageshack.us/img103/4804/ykprobabilityld7.png

Tree diagram