How many smurfs are there?

Question

Papa Smurf gathered some berries from the forest to the village and smurfs ate all those berries:

• The smurf who ate the most actually ate one-fourth of the berries eaten by the rest of the smurfs.
• The smurf who ate the third-most actually ate one-ninth of the berries eaten by the rest of the smurfs
• The smurf who ate the least actually ate one-tenth of the berries eaten by the rest of the smurfs.

How many smurfs are there?

Reference: Bilim ve Teknik Dergisi 2018-08

Answer 1

Let $N$ be the number of smurfs.

The information we are given implies that one smurf ate $1/5$ of the total berries, and that each of the other $N-1$ smurfs ate at least $1/11$ of the total berries. This means we must have $$ \frac{1}{5}+(N-1)\frac{1}{11} \leq 1, $$ and so $N\leq 9$.

The the smurf who ate the second-most at strictly less than $\frac{1}{5}$ of the total, and each of the $N-2$ smurfs (other than those who at the most and second most) ate at most $\frac{1}{10}$ of the total. This means we must have $$ \frac{1}{5}+\frac{1}{5}+(N-2)\frac{1}{10}>1, $$ and so $N\geq 9$.

Taken together, these bounds imply there must be $9$ smurfs.

Answer 2

I have an answer that works, but no proof of uniqueness yet.

There are 9 smurfs. One way this works is for there to be 1100 berries. The number of berries that each smurf eats is 220, 135, 110, 109, 108, 107, 106, 105, 100. The numbers sum to 1100. The greatest value, 220, is one-fourth of the remaining sum (1100-220=880). The third greatest value, 110, is one-ninth of the remaining sum (1100-110=990). The smallest value, 100, is one-tenth of the remaining sum (1100-100=1000).

Answer 3

"The smurf who ate the most actually ate one-fourth of the berries eaten by the rest of the smurfs." -> This sentence implies that the smurf who ate the most ate 1/5 of the total berries. Now, even if all the smurfs were eating 1/5th of the total berries each, even then we would have 5 smurfs . But we note that there are some smurfs that ate less than 1/5th of the total berries. This implies that there have to be more than 5 smurfs i.e there are at least 6 smurfs.

"The smurf who ate the third-most actually ate one-ninth of the berries eaten by the rest of the smurfs" -> So the smurf who ate the third-most actually ate one-tenth of the total berries. So someone ate between 1/6 and 1/10.

"The smurf who ate the least actually ate one-tenth of the berries eaten by the rest of the smurfs" -> So everyone ate more than 1/11.

Let x the quantity eaten by the second-most (1/6 > x > 1/10). Let y the quantity eaten by the N-4 others smurfs ( (N-4)/11 < y < (N-4)/10 )

So the total quantity is x + y + 1/6 + 1/10 + 1/11 And so, we search N so that

1/6 + (N-4)/10 + 1/6 + 1/10 + 1/11 > 1 > 1/10 + (N-4)/11 + 1/6 + 1/10 + 1/11

So, N=9. There are 9 smurfs.

Answer 4

I understood the answer to this question from the answer given by @user51199 . His answer has some mistakes though, so I am writing my own version of the answer below :

Let us reasonably assume that the smurf who ate the second most berries

1. ate lesser berries than the one who ate the most, and
2. more berries than the one who ate the third most.

Let us assume that there are a total of N smurfs.

Since the smurf who ate the second most, has to have had eaten less than 1/5th of the total berries, therefore :

$1/5 + 1/5$ + $1/10$ + (N-4) * $1/10$ + $1/11$ > 1

Therefore, N > 8

Also, the smurf who ate the second most berries ate more than $1/10$th of the total berries. Therefore :

$1/5 + 1/10 + 1/10$ + (N-4)*$1/11$ + 1/11 < 1

Therefore, N < 9.5

This means that 8 < N < 9.5 . Therefore , N = 9

Please feel free to ask for clarifications by commenting below.