Caesar Mathematics : I

Algebra Level 3

"Ape alone... weak. Apes together... strong." - Caesar


The Apes are gathering resources for the upcoming winter. This time, Caesar and others went gathering wood. To store the gatherings, Maurice - a wise ape, and Caesar's best friend - has devised a plan on how the resources should be distributed:

All the logs are to be divided into n n piles. After this, 1 n \frac{1}{n} -th of the logs from the first pile are to be shifted into the second pile. Then, 1 n \frac{1}{n} th of the logs in the second pile, after the first transfer, are to be shifted into the third pile. Next, 1 n \frac{1}{n} th of the logs in the third pile, after the transfer from the second pile, are to be shifted to the fourth, etc...

Finally, from the n n -th pile, 1 n \frac{1}{n} th of the logs in the n n -th pile, after the previous transfer, are to be shifted to the first pile. After that, each pile will have A A -amount of logs.

Caesar and Maurice agreed to divide the resources into 101 101 piles.

If, before shifting, the amount of logs in piles 1, 2, and 3 combined is 150 150 , how many logs were gathered?


The answer is 5050.

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2 solutions

John M.
Sep 20, 2014

Let the first pile initially have x 1 x_1 logs, the second pile x 2 x_2 , etc.; the last ( n n th) pile will thus have x n x_n .

The first pile is in a special position because 1 n \frac{1}{n} logs are initially withdrawn from it, and at the end of the process is it replenished in from the n n th pile, whereas all other piles are first filled with logs from the previous piles, and only then have 1 n \frac{1}{n} th of the logs withdrawn from them.

So, let us observe any pile but the first pile. Let's call our pile k k . Initially, it had x k x_k amount of logs, and a certain amount of logs were added to it - let's call it y y - from the ( k 1 ) (k-1) th pile. Then, from the overall sum of logs of y + x k y+x_k logs was withdrawn a 1 n \frac{1}{n} th part. So we're left with ( y + x k ) n 1 n (y+x_k)\frac{n-1}{n} logs (in that pile). Thus,

( y + x k ) n 1 n = A . ( 1 ) (y+x_k)\frac{n-1}{n}=A. \rightarrow (1)

In the previous - ( k 1 ) (k-1) th pile, if it wasn't the first pile (i.e. if k 2 k \neq 2 ), it must've had A A logs. (The first pile only acquires A A amount of logs after it is replenished from the n n th pile). Therefore, before the transfer of y y logs it held A + y A+y logs. By the condition the withdrawn amount of y y logs constitutes 1 n \frac{1}{n} th part of A + y A+y , i.e.

y = 1 n ( A + y ) . ( 2 ) y=\frac{1}{n}(A+y). \rightarrow (2)

From here we find y = 1 n 1 A . y=\frac{1}{n-1}A. Substituting this into the first expression, we obtain x k = A x_k=A .

Hence, in every pile (maybe with an exception of first and second piles - prior to being excluded from consideration), there were initially A A logs:

x 3 = x 4 = . . . = x n = A . ( 3 ) x_3=x_4=...=x_n=A. \rightarrow (3)

The unknown x 1 x_1 can be found in a following way; by the condition, x 1 x_1 has 1 n \frac{1}{n} th of its logs withdrawn from it. We're left with x 1 n 1 n x_1\frac{n-1}{n} logs. At the end of the process, a certain amount of logs - y y - is added to it from the last pile. Hence,

y + x 1 n 1 n = A . ( 4 ) y+x_1\frac{n-1}{n}=A. \rightarrow (4)

Applying the same reasoning to the n n th pile, we find that y = 1 n 1 A y=\frac{1}{n-1}A . Inserting this into expression ( 4 ) (4) we obtain

x 1 = ( n 2 ) n ( n 1 ) 2 A . ( 5 ) x_1=\frac{(n-2)n}{(n-1)^2}A. \rightarrow (5)

To investigate for x 2 x_2 we dig up

( 1 n x 1 + x 2 ) n 1 n = A , ( 6 ) \left(\frac{1}{n}x_1+x_2 \right)\frac{n-1}{n}=A, \rightarrow (6)

where x 1 x_1 is determined by expression ( 5 ) (5) . Solving this equation, we obtain

x 2 = n ( n 1 ) ( n 2 ) ( n 1 ) 2 A . x_2=\frac{n(n-1)-(n-2)}{(n-1)^2}A.

And thus,

x 1 = n 2 2 n ( n 1 ) 2 A ; \boxed{x_1=\frac{n^2-2n}{(n-1)^2}A}; x 2 = n 2 2 n + 2 ( n 1 ) 2 A ; \boxed{x_2=\frac{n^2-2n+2}{(n-1)^2}A;} x 3 = x 4 = . . . = x n = A . \boxed{x_3=x_4=...=x_n=A.}

CAESAR CAESAR

these apes are too legit

Julian Liu - 6 years, 8 months ago

@John Muradeli : Requesting a solution, or at least the number of logs initially in the piles.

Jon Haussmann - 6 years, 8 months ago

I think there is issue with the problem statement.

The problem states that:

Finally, from the n n -th pile, 1 n \frac{1}{n} th part of the logs that were moved there in that last transfer are shifted to the first pile.

This means nothing was moved out from the original logs in n t h n^{th} pile. The movement happened only from the logs that were moved to the n t h n^{th} pile from the ( n 1 ) t h (n-1)^{th} pile.

However, looking at your solution, it seems that the n t h n^{th} pile case is no different than ( n 1 ) t h (n-1)^{th} pile case.

Hence, it should have been mentioned like this:

Finally, from the n n -th pile, 1 n \frac{1}{n} th of the logs, after transfer from the ( n 1 ) t h (n-1)^{th} pile, are shifted to the first pile.

Pawan Kumar - 6 years, 2 months ago
Ben C
Nov 17, 2014

"Caesar and Maurice agreed to divide the resources into 101 piles" All piles have the same amount of logs.

"If, before shifting, the amount of logs in plies 1, 2, and 3 combined is 150..." Piles 1, 2, and 3 have a combined number of 150 logs.

Each pile has 150 3 = 50 \frac{150}{3}=50 , so 101 piles would be 50 101 = 5050 50*101=5050

Ah - clever smarty-pants, eh?

You found quite the loophole in my problem.

(And I hope you did manage to solve it the way it was supposed to be solved as well).

John M. - 6 years, 6 months ago

Log in to reply

Simpler is better? Anyway, I'll try it the other way too. Thanks.

ben c - 6 years, 6 months ago

Note that it is not stated that "all piles have the same amount of logs" at the start.

Calvin Lin Staff - 6 years, 2 months ago

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