Puzzle 53 | The Counterfeit Coin
A box contains n coins, of which 7 of them are counterfeit with tails on both sides and the rest are fair coins. If one coin is selected from the bag and tossed, the probability of getting a tail is 17/20. Find the value of ‘n’.
Answer: n = 10
Probability of getting a tail can be in two ways
1) Selecting any of those 7 coins with tails on two side is = (7/n)
2) Selecting any one, other than those 7 biased coins is (n-7)/n
(7/n)(1) + (n-7/n)*(1/2) = 17/20 | | | | | | probability of probability of getting a tail from getting a tail from biased coins fair coins
solving above equation, we get n = 10
This Puzzle is contributed by Harika Mulumudi. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.