In a trip, there are 12 boys and 8 girls. 5 of the boys and 4 of the girls wear
glasses. 2 students are selected at random to serve as the monitors.
(1) Find the probability that the two monitors wear glasses.
(2) Find the probability that the two monitors are of opposite sex.
probability
2007-10-18 7:24 am
回答 (3)
2007-10-18 9:54 am
✔ 最佳答案
(1) P (two monitors wear glasses)
= (5 + 4) / (12 + 8)
= 9 / 20
(2)
Method 1
P (two monitors are of opposite sex)
= P (boy) x P (girl | boy is selected) + P (girl) x P (boy | girl is selected)
= (12/20)x(8/19) + (8/20)x(12/19)
= 24/95 +24/95
= 48/95
Method 2
P (two monitors are of opposite sex)
= 1 - P (two monitors are of same sex)
= 1 - (12/20)x(11/19) - (8/20)x(7/19)
= 48/95
參考: My Maths knowledge
2007-10-18 11:42 am
1)
P(two monitors wear glasses)
=P(first monitor wear glasses and 2nd monitor wear glasses)
=(9/20)X(8/19)
=18/95
2) P(two monitoirs are of oppsite sex)
=P(first monitor is boy and 2nd monitor is girl)+P(first monitor is girl and 2nd monitor is boy)
=(12/20)x(8/19)+(8/20)X(12/19)
=48/95
P(two monitors wear glasses)
=P(first monitor wear glasses and 2nd monitor wear glasses)
=(9/20)X(8/19)
=18/95
2) P(two monitoirs are of oppsite sex)
=P(first monitor is boy and 2nd monitor is girl)+P(first monitor is girl and 2nd monitor is boy)
=(12/20)x(8/19)+(8/20)X(12/19)
=48/95
2007-10-18 7:36 am
(1) P (two monitors wear glasses) = (5 + 4) / (12 + 8)
= 9 / 20
(2) P (two monitors are of opposite sex) = 5/12 X 4/8
= 5 / 24
2007-10-21 13:32:39 補充：
2) P (two monitors are of opposite sex) = 5/12 X 4/8 X 2 = 5 / 12
= 9 / 20
(2) P (two monitors are of opposite sex) = 5/12 X 4/8
= 5 / 24
2007-10-21 13:32:39 補充：
2) P (two monitors are of opposite sex) = 5/12 X 4/8 X 2 = 5 / 12
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