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Nov 11, 2019
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Nov 11, 2019
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Nov 11, 2019
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I have a solution to this that is very involved. But I will try to outline it even though it may not be the most elegant solution. Here it goes:

Here I assume that x is the number of people stranded on the island, n is the number of coconuts gathered and piled before dark, and k1, k2, k3, k4, and k5 are the number of coconuts, respectively, in each of the x piles after the additional coconut is given to the monkey and before the night watches 1 through 5 hide their own portion. We can write a system of 5 equations involving the 7 variables(n, x,  k1, k2, k3, k4, and k5) as follows:

\(k_1x= n -1\)

\(k_2x=n-k_1-2 \)

\(k_3x=n-(k_1+k_2)-3\)

\(k_4x=n-(k_1+k_2+k_3)-4\)

\(k_5x=n-(k_1+k_2+k_3+k_4)-5\)

 

This system evidently has an infinite number of solution since the number of variables is larger than the number of equations and can be approached in multiple ways, but all methods would involve a reduction of the number of variables. Here we are asked to find the minimum value of n for a given value of x, so we will reduce the number of equations to only one, a quintic polynomial equation in the variable x, the number of stranded people, and containing two other variables, n and k5. I will only do step one of this process since the rest can be accomplished similarly.

Multiply the 5th equation by x to get:

\(k_5x^2=nx-(k_1+k_2+k_3+k_4)x-5x\)

Replace k1x, k2x, k3x, and k4x by their values given by the 1st through 4th equation in the system to get :

\(k_5x^2=nx-[(n-1)+(n-k_2-2)+(n-k_1-k_2-3)+(n-k_1-k_2-k_3-4)]-5x\)

\(=nx-4n+10+3k_1+2k_2+k_3-5x\)

Note that k4 has been eliminated from the equation and by repeating this process 3 more times , we can also eliminate k1, k2, and k3 to get the quintic we were aiming at:

 

\(k_5x^5=(n-5)x^4+(10-4n)x^3+(6n-10)x^2+(5-4n)x+(n-1)\)

 

Moving all terms to the left side we get:

 

\(k_5x^5+(5-n)x^4+(4n-10)x^3+(10-6n)x^2+(4n-5)x+(1-n)=0\)

 

Well this is the formula we were looking for. The author of this question claims that if the there are only six people stranded on the island, the initial number of coconuts would have to be a minimum of 7,771. Let's see if this holds true. Substituting 6 for x in the above equation we get:

\(0=7,776k_5+1,296(5-n)+216(4n-10)+36(10-6n)+6(4n-5)+(1-n)\)

\(7,776k_5+4,651-625n\), which implies that

 

\(n=\frac{7,776k_5+4,651} {625}\)

 

Here we need to find the smallest value of k5 that results in an integer value for n. I gave the problem to EXCEL to solve. EXCEL did a search for k5=1, 2, ..., 10,000, we decided to limit the search, and came up with 16 values for k5 that resulted in integer values for n.The smallest of these values was k5=624 and the largest was k5=9,999. Plugging these for k5 in the formula for n, gave us, what do you know, n= 7,771 and n= 124,411.

Nov 11, 2019

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