Probability

P(having a girl) = 1/2

P(having at least 4 children)
= P (the first three babies being girls)
= (1/2)³
= 1/8

2010-04-15 21:58:44 補充：
Alternative method :

P(having at least 4 children)
= P(4 children) + P(5 children) + P(6 children) + ...... P(infinitive children)
= (1/2)^4 + (1/2)^5 + (1/2)^6 + ...... + infinitive term
= (1/2)^4/[1 - (1/2)]
= (1/2)
= (1/2)^4/(1/2)
= (1/2)^3
= 1/8

The chance of having a baby boy or a baby girl is the same. Mrs. Wong
will stop having more babies once she gets a boy. What is the
probability that she will have at least 4 children?
P(she will have at least 4 children)
= (1/2)^3 + (1/2)^4 + (1/2)^5 + ... + (1/2)^(inf.)
= (1/2)^3 / (1-1/2)
= (1/8)/(1/2)
= 1/4


2010-04-16 08:56:18 補充：
The answer is wrong,
因為如果佢頭3個係女的話,無論第4個係唔係女,都一定會生,咁既話,就一定有4個小朋友.
所以第一個terms, 係 (1/2)^3