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CBSE Class 12-science Answered

Two cards from a pack of 52 cards are lost . From the remaining 50 cards , one card is drawn . If the drawn card is a spade , what is the probability that lost cards were both spades ? 
Asked by abhitailor158 | 17 Mar, 2019, 12:12: PM
answered-by-expert Expert Answer
There are three possible events
 
(1) Both the lost cards are spade . Let this event be E1 . probability of this event ,  p( E1 ) = (13/52) (12/51)
 
(2) one lost card is spade and other one is not spade. Let this event be E2 . Probability of this event,
 
                     p( E2 ) = (13/52) (39/51) + (39/52) (13/51) = (26×39) / (52 × 51 )
 
(3) both the lost cards are not spade. Let this event be E3 . probability of this event, p(E3 ) = (39/52) (38/51)
 
probability of getting spade from 50 cards , when both lost cards are spade ,  p(S/E1 ) = 11/50
 
probability of getting spade from 50 cards , when one lost card is spade and other one is not a spade,  p(S/E2 ) = 12/50
 
probability of getting spade from 50 cards , when both lost cards are not spade ,  p(S/E3 ) = 13/50
 
If spade is selected from 50 cards after two cards were lost,
then the probability that two lost cards are spade is  p(E1 / S ) and it is obtained by applying Bayes theorem
 
begin mathsize 12px style p left parenthesis space E subscript 1 space divided by S right parenthesis space equals space fraction numerator P left parenthesis space E subscript 1 space right parenthesis space p left parenthesis space S divided by E subscript 1 right parenthesis over denominator P left parenthesis space E subscript 1 space right parenthesis space p left parenthesis space S divided by E subscript 1 right parenthesis space plus space P left parenthesis space E subscript 2 space right parenthesis space p left parenthesis space S divided by E subscript 2 right parenthesis space plus space P left parenthesis space E subscript 3 space right parenthesis space p left parenthesis space S divided by E subscript 3 right parenthesis end fraction
p left parenthesis space E subscript 1 space divided by S right parenthesis space equals space fraction numerator begin display style fraction numerator 13 cross times 12 over denominator 52 cross times 51 end fraction end style cross times begin display style 11 over 50 end style over denominator fraction numerator 13 cross times 12 over denominator 52 cross times 51 end fraction cross times 11 over 50 space plus space fraction numerator 26 cross times 39 over denominator 52 cross times 51 end fraction cross times 12 over 50 space plus fraction numerator 39 cross times 38 over denominator 52 cross times 51 end fraction cross times 13 over 50 end fraction space almost equal to space 0.075
end style
Answered by Thiyagarajan K | 17 Mar, 2019, 10:51: PM
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