A Cliche Test Problem

The number 25 can be thought of the sum of a few positive integers.

Determine the largest possible product of these integers.


The answer is 8748.

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2 solutions

Francis Kong
Mar 24, 2018

Let's start by eliminating the strictly dominated strategies:

  • For any number n 1 n \ge 1 , 1 ( n 1 ) < n 1(n-1)<n
  • For any number n 5 n \ge 5 , 2 ( n 2 ) > n 2(n-2)>n and 3 ( n 3 ) > n 3(n-3) >n
  • For n = 4 n =4 , 2 × 2 = 4 2 \times 2 = 4 and 1 × 3 < 4 1 \times 3 < 4 , so 2 + 2 2+2 and 4 can be used interchangeably.

So to maximize the product of the partition that adds to 25, each of the integer will have to be either 2, 3 or 4

  • If most of them are 2's you can have 2 + 2 + 2 + 2 + 2 + 2 + 2 + 2 + 2 + 2 + 2 + 3 = 25 2+2+2+2+2+2+2+2+2+2+2+3 = 25 and the product of them amounts to 2 11 × 3 = 6144 2^{11} \times 3 = 6144
  • If most of them are 3's you can have 3 + 3 + 3 + 3 + 3 + 3 + 3 + 4 = 25 3+3+3+3+3+3+3+4 = 25 and the product of them amounts to 3 7 × 4 = 8748 3^{7} \times 4 = 8748
  • If most of them are 4's you can have 4 + 4 + 4 + 4 + 4 + 2 + 3 = 25 4+4+4+4+4+2+3 = 25 and the product of them amounts to 4 5 × 2 × 3 = 6144 4^{5}\times 2 \times 3 = 6144
  • If half of them are 3's and half of them are 2's you have 2 + 2 + 2 + 2 + 2 + 3 + 3 + 3 + 3 + 3 = 25 2+2+2+2+2+3+3+3+3+3 = 25 and the product of them amounts to 2 5 × 3 5 = 7776 2^{5} \times 3^{5} = 7776

As such the answer is 8748 \boxed{8748}

Giorgos K.
Mar 22, 2018

Mathematica

Max[Times@@@IntegerPartitions@25]

returns 8748

the numbers are

{4, 3, 3, 3, 3, 3, 3, 3} OR {3, 3, 3, 3, 3, 3, 3, 2, 2}

Using a calculator doesn't count as a solution. I want to know the process.

Blan Morrison - 3 years, 2 months ago

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You asked something and I gave you the answer and I also showed you how I got this answer. Next time you should denote "Using a calculator doesn't count as a solution" or pick some big numbers that a calculator cannot compute by brute-force. Otherwise you should accept all the answers like calculators or even "google search" because those are tools that many people use nowadays. For example if you had picked a number like 250 instead of 25 then it would be much much harder for a computer to solve because the number of partitions is massive.Puzzle-making is not an easy task.

Giorgos K. - 3 years, 2 months ago

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Dude, calm down. I'm not even the one who made the puzzle. I'm just here to learn.

Blan Morrison - 3 years, 2 months ago

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@Blan Morrison I am calm and I'm glad that you learned something about todays problem solving.

Giorgos K. - 3 years, 2 months ago

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