hi,
probability of choosing the coin with both heads is
1/(n+1)
probability that it gives head is
[1/(n+1)](1) ......(1)
probability of choosing the rest of the coins is
[n/n+1]
probability that the chosen coins give head is
[n/n+1](1/2) ......(2)
adding (1) and (2) and equating to 7/12
we get n as 5
and n+1 as 6.
Moderation Team
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