Can You Solve the Passcode Riddle?


Here's a nice riddle. Three members of a team have been captured. On their way into the prison (guarded by ravenous mutant salamanders), they pass a series of numbered doorways, each with a keypad featuring the numbers one through nine. Each keypad opens with a code...but you have no idea what that code might be. One member will be allowed to try to escape by facing a challenge. Can the remaining two listen in on that challenge, figure out the correct hallway, and figure out the passcode to open it? With some basic math, they can succeed.

Some more details: Zara, the team member participating in the challenge, has a one-way audio transmitter that allows the other two team members to listen. As Zara is led to the challenge through one of the hallways, she is informed that her challenge is to guess the passcode for her hallway based on rules. Zara is told that the passcode will contain three positive whole numbers, in ascending order (like 1, 2, 3—the second number is greater than or equal to the first, the third likewise to the second).

Zara is told that she may ask up for up to three clues about the code—but she can't say anything else, or else she too will be fed to the mutant salamanders! Through this process of requesting clues and thinking through the problem, Zara implicitly passes information to her compatriots about what the answer is.

Zara asks for the first clue, and is told that the product of the three numbers in the code (x * y * z) is 36. Zara asks for the second clue, and is told that the sum of the numbers in the code (x + y + z) is the same as the number of the hallway she entered. There is a long silence. Then she asks for the third clue, and is told that the largest (greatest) number appears only once in the combination. Shortly after, Zara punches in the code and escapes.

Given that information, can you figure out the passcode? The video below walks through the puzzle and its solution. Here's the text of the puzzle in its simplest form, transcribed from the video (it helpfully asks you to pause before explaining the solution!):

Find three numbers in ascending order!

1. The product of the three numbers is 36.

2. The sum of the three numbers is the same as Zara's hallway number, which you don't know but she does.

3. The largest number must be unique.

4. Zara, a perfect logician, needed clues 1-3 to escape.

Start your figuring!

Then tune in to the video to test your solution:

For a bit more on this puzzle, check out this TED-Ed page, especially the "Dig Deeper" section which contains links to various math resources that you may find useful. (It also includes a link to a puzzle variant that may be interesting after you solve this one.)

Honey Bees Can Understand the Concept of Zero

The concept of zero—less than one, nothing, nada—is deceptively complex. The first placeholder zero dates back to around 300 BCE, and the notion didn’t make its way to Western Europe until the 12th century. It takes children until preschool to wrap their brains around the concept. But scientists in Australia recently discovered a new animal capable of understanding zero: the honey bee. According to Vox, a new study finds that the insects can be taught the concept of nothing.

A few other animals can understand zero, according to current research. Dolphins, parrots, and monkeys can all understand the difference between something and nothing, but honey bees are the first insects proven to be able to do it.

The new study, published in the journal Science, finds that honey bees can rank quantities based on “greater than” and “less than,” and can understand that nothing is less than one.

Left: A photo of a bee choosing between images with black dots on them. Right: an illustration of a bee choosing the image with fewer dots
© Scarlett Howard & Aurore Avarguès-Weber

The researchers trained bees to identify images in the lab that showed the fewest number of elements (in this case, dots). If they chose the image with the fewest circles from a set, they received sweetened water, whereas if they chose another image, they received bitter quinine.

Once the insects got that concept down, the researchers introduced another challenge: The bees had to choose between a blank image and one with dots on it. More than 60 percent of the time, the insects were successfully able to extrapolate that if they needed to choose the fewest dots between an image with a few dots and an image with no dots at all, no dots was the correct answer. They could grasp the concept that nothing can still be a numerical quantity.

It’s not entirely surprising that bees are capable of such feats of intelligence. We already know that they can count, teach each other skills, communicate via the “waggle dance,” and think abstractly. This is just more evidence that bees are strikingly intelligent creatures, despite the fact that their insect brains look nothing like our own.

Considering how far apart bees and primates are on the evolutionary tree, and how different their brains are from ours—they have fewer than 1 million neurons, while we have about 86 billion—this finding raises a lot of new questions about the neural basis of understanding numbers, and will no doubt lead to further research on how the brain processes concepts like zero.

[h/t Vox]

Can You Solve This Ice Cream Cone Riddle?

How much is an ice cream cone worth? In this visual riddle by Budapest-based artist Gergely Dudás (who posts comics on, the answer requires a little math.

The riddle asks you to determine how much an ice cream cone, a scoop of white-colored ice cream (let’s call it vanilla), and a scoop of pink-colored ice cream (let’s call it strawberry) are worth, according to the logic of the puzzle.

Stare at the equations for a while, then scroll down for the answer.

A math riddle that asks you to figure out what numbers each ice cream cone or scoop represents
Gergely Dudás


Are you sure?

OK, let's walk through this.

Three ice cream cones multiplied together are equal to the number 27. Since 3 multiplied by 3 multiplied by 3 equals 27, each cone must be equal to 3.

Moving on to the next row, two ice cream cones each topped with a scoop of vanilla ice cream added together equal 10. So since each cone equals 3, the vanilla scoops must each equal 2. (In other words, 3 plus 3 plus 2 plus 2 equals 10.)

Now, a double scoop of vanilla on a cone plus a single scoop of strawberry on a cone equals 11. So if a double-scoop of vanilla equals 4 (2 plus 2) and each cone is equal to 3, the strawberry scoop must equal 1. (Because 4 plus 6 equals 10, plus 1 for the strawberry scoop equals 11.)

And finally, one vanilla scoop on a cone, plus one empty cone, plus a double-scoop of strawberry and a single scoop of vanilla on a cone, all together equals 15. One scoop of vanilla on a cone is equal to 5 (2 plus 3), and an empty cone is equal to 3. Two strawberry scoops plus one vanilla scoop plus one cone can be calculated as 1 plus 1 plus 2 plus 3 (which comes out to 7). So together, one vanilla scoop (5) plus one cone (3) plus a triple scoop with two strawberries and one vanilla on a cone (7) equals 15.

And there you have it.

A cartoon-style legend that shows that one cone equals 3, one white scoop equals 2, and one pink scoop equals 1.
Gergely Dudás

If frozen dairy-themed challenges are your thing, he also has a hidden image puzzle that challenges you to find the lollipop in a field of ice cream cones. Check out more of his work on his website and Facebook.


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