Answer: P(B) = 7/20
Work Shown:
Given info
P(A) = 7/20
P(A∩B)=49/400
P(B) = unknown
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P(A∩B) = P(A)*P(B), assuming A and B are independent events
49/400 = (7/20)*P(B)
(7/20)*P(B) = 49/400
P(B) = (20/7)(49/400)
P(B) = (20*49)/(7*400)
P(B) = (20*7*7)/(7*20*20)
P(B) = 7/20
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Note how
P(A∩B) = P(A)*P(B)
P(A∩B) = (7/20)*(7/20)
P(A∩B) = (7*7)/(20*20)
P(A∩B) = 49/400
which helps to confirm the answer.