Like that will ever happen!

There are n + m n+m coins in a bag. In the bag n n of them are fair (equal probability of outcomes) and m 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 11 17 \frac{11}{17} . Given that n n and m m are positive and that 2 n 133 + m 2n\le133+m work out how many different amounts of coins there could be.


The answer is 7.

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2 solutions

First let A A be the event that a fair coin is chosen at random. Then we are given that

P ( A ) ( 1 2 ) + P ( A ) ( 1 ) = 11 17 P(A)*(\frac{1}{2}) + P(\overline{A})*(1) = \frac{11}{17}

( n n + m ) ( 1 2 ) + ( m n + m ) ( 1 ) = 11 17 \Longrightarrow (\frac{n}{n + m})(\frac{1}{2}) + (\frac{m}{n + m})(1) = \frac{11}{17}

17 ( n + 2 m ) = 22 ( n + m ) 12 m = 5 n \Longrightarrow 17*(n + 2m) = 22*(n + m) \Longrightarrow 12m = 5n .

So m = ( 5 12 ) n m = (\frac{5}{12})n . Substituting this into the given inequality gives us that

2 n 133 + ( 5 12 ) n ( 19 12 ) n 133 n 84 2n \le 133 + (\frac{5}{12})n \Longrightarrow (\frac{19}{12})n \le 133 \Longrightarrow n \le 84 .

But since 12 12 and 5 5 are coprime, the fact that 12 m = 5 n 12m = 5n implies that n n must be a multiple of 12 12 . Thus since 84 = 7 12 84 = 7*12 , there are 7 7 possible values for n n with 7 7 corresponding values for m m , resulting in 7 \boxed{7} different values for n + m n + m .

(Note that since n n increases by increments of 12 12 and m m increases by increments of 5 5 we have that n + m n + m increases by increments of 17 17 , going from a minimum of 17 17 to a maximum of 119 119 coins in the bag.)

what a coincidence!!I did it the same way!!

Adarsh Kumar - 6 years, 8 months ago

i thought it to be 119 , my approach was right but got it wrong!

subham jyoti mishra - 6 years, 8 months ago
Hs N
Oct 15, 2014

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 m 2 ( m + n ) \frac{2m+n}{2(m+n)} = 1 -\frac{m}{2(m+n)} . Since this equals 11 17 \frac{11}{17} , we know that m 2 ( n + m ) = 6 17 \frac{m}{2(n+m)}=\frac{6}{17} , which reads 5 n = 12 m 5n=12m after rearranging. Therefore the only possible solutions are for n n a multiple of 12 12 .

Now we need to use the inequalit. We can multiply it by 12 12 to obtain 24 n 12 133 + 12 m = 12 133 + 5 n 24n\leq 12\cdot133 + 12m = 12\cdot133 + 5n , and thus 19 n 12 133 19n\leq 12\cdot133 , or n 84 n\leq84 . Since 84 = 7 12 84=7\cdot12 , there are exactly 7 \boxed{7} solutions where m , n m,n are positive.

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