Answer :


Total number of students = 80


i. Probability of both being girl.


For both being a girl i.e. girl is selected from class 10A and another girl is selected from class 10B


Total number of girls = 45


Probability of both being girls




ii. Probability of both being boys


For both being a boy i.e. boy is selected from class 10A and another boy is selected from class 10B


Total number of boys = 35


Probability of both being boys




iii. Probability of one boy and one girl


For one boy and one girl, a boy is selected from class 10A and a girl from class 10B or vice a versa.


i.e. P (boy from class 10A) × P (girl from class 10B) +


P (girl from class 10A) × P (boy from class 10B)





= 0.50


Probability of one boy and one girl is 0.50.


iv. Probability of at least one boy.


For at least one boy means one boy and another girl, or both boy.


In this, one boy and other girl can occur in two ways:


1st boy and 2nd girl or the other way round.


(20 × 25) + (20 × 15)


= 500 + 300


= 800


the possibility is 0.50 (obtained in previous part)


We have already seen the possibility of both the boys in above solved parts i.e. probability of both boys is 0.4375


Probability of at least one boy = 0.50 + 0.4375


= 0.9375


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