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So a picture appeared on FB asking for the answer to the picture below, to which many people responded "14".

enter image description here

Everywhere I looked, people answered 14. However, I stood out because I was the only one who said that the answer is 11.25.

The problem can be represented algebraically like this:

4x = 5

x = 1.25

9x = ?

9(1.25) = 11.25

So my question is..how the heck did people get 14? Am I using the wrong approach?

chowching
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GelatinFox
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    Well if you are just asking why they got 14, I guess Jared has already answered that. But you could also argue that the answer is 11.25. It depends on how you view the puzzle. I could say that the answer is 8 and have my own reasoning why. – chowching Sep 30 '13 at 01:55

5 Answers5

7

The first picture shows $5$ squares, the $4$ smaller ones, along with the $2\times 2$ square. Counting $1\times 1$, $2\times 2$, and $3\times 3$ squares, the second picture shows $14$ squares.

With this reasoning, a $4\times 4$ grid would equal $30$.

enter image description here

In general, an $n\times n$ grid would equal $1^2+2^2+\ldots+n^2=\frac{n(n+1)(2n+1)}{6}$.

These are the square pyramidal numbers, because they are the number of cubes needed to build a pyramid with a square base $n$ levels high.

Community
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Jared
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  • Would my answer be incorrect? The problem was very vague. I would say that 11.25 is another possible answer from an algebraic standpoint. – GelatinFox Sep 30 '13 at 01:44
  • btw, I'm voting your answer as correct. Just waiting for the time – GelatinFox Sep 30 '13 at 01:50
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    @Guest123456: You've given a fine answer. The problem is not at all well-posed. I was just explaining what I believe to be the reasoning behind the common answer of $14$. I'm sure there are ways to reason that the answer should be one of many different numbers. – Jared Sep 30 '13 at 01:53
  • Thank you, for the answer. Always learn something new here :) – GelatinFox Sep 30 '13 at 01:53
  • @Guest123456: The question is vague, and your answer is valid. (Another "wrong" ---but perfectly valid--- approach would be to set up a proportion such that $5$ is the total length of the line segments.) The puzzle would be better-posed if it included a $1\times 1$ square with answer "$1$", to help rule out what the puzzler considers "wrong" answers. (And, really, the puzzler shouldn't call alternative answers wrong; instead, he should say, simply, "That's not the solution I had in mind.") – Blue Sep 30 '13 at 01:58
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I see the answer being the total number of squares, regardless of their size, but squares. Of those I see 14. 9 small squares 4 squares (overlapping) of 4 squares each ( 1,2,4,5 - 2,3,5,6 - 4,5,7,8 - 5.6.8.9) and 1 large square.

Corrie
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Think it simply. Count the total amount of squares.

Asd
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My answer is 20. If the first image... or pattern is valued at five then the second image has four patterns inside of it that equal 5. So 4×5=20. I believe this is correct but i also believe that there is more than 1 correct answer to the riddle

Bruce
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I could say it is numbering the smaller boxes from $1$ like below and then adding the diagonal from top left to top right and still be correct.

enter image description here

Note that this is an $AP$ and can be generalized to

$$\cfrac{n(n^2 + 1)}2$$

user2798694
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