probability

Let x be the required number of children :P(at least a son and a daughter among x children) > 0.91 - P(x boys) - P(x girls) > 0.91 - (1/2)^x - (1/2)^x > 0.90.1 > (1/2)^(x - 1)log 0.1 > (x - 1) log (1/2)- 1 > (x - 1) (- 0.30103) 3.322 < x - 1x > 4.322Mrs Lee should give birth to 5 children.

2011-06-06 18:02:12 補充：
一仔一女的反面是全仔或全女,

所以 一仔一女機會 + 全仔或全女機會 = 1

1 - 全仔或全女機會 = 一仔一女機會