Vincen wrote:A box contains four coins, of which two coins have heads on both their faces, one coin has tail on both its faces and the fourth coin is a normal one. A coin is picked at random and then tossed. If head is the outcome of the toss, then find the probability that the other face (hidden face) of the coin tossed is also a head.
A. 2/5
B. 1/2
C. 4/5
D. 2/3
E. 3/4
Two coins have heads on both faces:
H, H
H, H
One coin has heads on only one face:
H, T.
One coin has tails on both faces:
T, T.
Since the outcome of the toss is heads, the flipped coin must be one of the 3 coins in blue.
The 3 coins in blue comprise a total of 6 faces:
H, H
H, H
H, T.
Since the outcome of the toss is heads, one of the H's above is facing upward on the toss.
Thus, one of the 5 remaining faces -- H, H, H, H, T -- must be facing downward.
Of these 5 remaining faces, 4 are heads.
Thus, P(downward face is heads) = 4/5.
The correct answer is
C.
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