There are n + m coins in a bag. In the bag n of them are fair (equal probability of outcomes) and m of them are double sided heads (heads on both sides).
The probability of picking a random coin and tossing it and getting heads is 1 7 1 1 . Given that n and m are positive and that 2 n ≤ 1 3 3 + m work out how many different amounts of coins there could be.
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what a coincidence!!I did it the same way!!
i thought it to be 119 , my approach was right but got it wrong!
The chance of head equals the total number of head, divided by the total number of sides of a coin, hence is 2 ( m + n ) 2 m + n = 1 − 2 ( m + n ) m . Since this equals 1 7 1 1 , we know that 2 ( n + m ) m = 1 7 6 , which reads 5 n = 1 2 m after rearranging. Therefore the only possible solutions are for n a multiple of 1 2 .
Now we need to use the inequalit. We can multiply it by 1 2 to obtain 2 4 n ≤ 1 2 ⋅ 1 3 3 + 1 2 m = 1 2 ⋅ 1 3 3 + 5 n , and thus 1 9 n ≤ 1 2 ⋅ 1 3 3 , or n ≤ 8 4 . Since 8 4 = 7 ⋅ 1 2 , there are exactly 7 solutions where m , n are positive.
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First let A be the event that a fair coin is chosen at random. Then we are given that
P ( A ) ∗ ( 2 1 ) + P ( A ) ∗ ( 1 ) = 1 7 1 1
⟹ ( n + m n ) ( 2 1 ) + ( n + m m ) ( 1 ) = 1 7 1 1
⟹ 1 7 ∗ ( n + 2 m ) = 2 2 ∗ ( n + m ) ⟹ 1 2 m = 5 n .
So m = ( 1 2 5 ) n . Substituting this into the given inequality gives us that
2 n ≤ 1 3 3 + ( 1 2 5 ) n ⟹ ( 1 2 1 9 ) n ≤ 1 3 3 ⟹ n ≤ 8 4 .
But since 1 2 and 5 are coprime, the fact that 1 2 m = 5 n implies that n must be a multiple of 1 2 . Thus since 8 4 = 7 ∗ 1 2 , there are 7 possible values for n with 7 corresponding values for m , resulting in 7 different values for n + m .
(Note that since n increases by increments of 1 2 and m increases by increments of 5 we have that n + m increases by increments of 1 7 , going from a minimum of 1 7 to a maximum of 1 1 9 coins in the bag.)