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Can You Solve the Virus Riddle?

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In this video riddle, you're a researcher who needs to solve a math problem. The fate of humanity is at stake!

Here's the setup. Your research group has isolated a lethal virus and is studying it in a lab. But one night after you leave the lab, an earthquake strikes and breaks the virus vials. This means that 15 of the 16 rooms in the lab are contaminated, and you have to get past the lab's security system in order to destroy the virus. (There is time pressure, as eventually the virus will escape the lab and kill us all!)

The lab is built as a 4x4 grid, containing a total of 16 rooms, with an entrance at the northwest corner and an exit at the southeast corner. Each room is connected to the adjacent rooms by an airlock. Only the entrance and exit rooms are connected to the outside. The virus has been released in every room except the entrance room.

To destroy the virus samples, you must enter each room and pull its self-destruct switch, destroying the room and the virus within it. But there's a problem—because the lab is in lockdown mode, once you enter a contaminated room, you can't exit without activating the self-destruct switch. Furthermore, once the self-destruct switch has been activated, you cannot re-enter a contaminated room.

Your job is to enter through the entrance room, exit through the exit room, and destroy the virus in every contaminated room. How can you do it?

From the video (at the 1:41 mark), here are the official rules and restrictions:

1. You must enter the building through the entrance and leave through the exit.

2. Every room except the entrance is contaminated.

3. Once you enter a contaminated room, you must pull the switch.

4. After pulling the switch, you must immediately leave the room.

5. You cannot return to a room after its switch has been activated.

Watch the video below for a visual explanation of the problem. This one's a bit of a forehead-smacker when you see the solution.

For more on this puzzle (and its solution), check out this TED-Ed page.

Note: If you're interested in math (without puzzle spoilers), this problem is related to Hamiltonian Paths, or paths that visit each point exactly once.

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Health
The Math Behind the Classic Eye Chart Is Surprisingly Complex
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Next time you're forced to take a vision exam at the DMV, take a moment to appreciate the complex math that went into the eye chart. What seems like a fairly straightforward way to assess eyesight is actually the result of specific calculations that can tell you a lot about how the human eye works.

As The Verge explains in the video below, eye charts measure one aspect of healthy vision: visual acuity. This is our ability to make out fine details in our surroundings—kind of like the resolution of a computer screen, but instead of pixels, it's measured in degrees. It's easy for our eyes to tell the difference between two points of light coming from different directions, but if those points start to move closer together they will eventually blur into one. The angle created just as two lines of light become too close for our eyes to distinguish them is called the resolution limit. In healthy adults, it measures one-sixtieth of a degree, or one arcminute wide.

When a doctor asks you to read an eye chart, the resolution limit is what they're looking for. The letters in the middle of an eye chart are all designed to be exactly one arcminute thick. If your vision is sharp, you should be able tell the difference between the white spaces and the black lines of the text from 20 feet away. 

A perfect 20/20 score on an eye chart test doesn't mean you have perfect vision: Visual acuity, along with color, contrast, and depth perception, are all important parts of healthy eyesight. And a higher resolution limit isn't always a sign of a permanent problem: For people who spend their days staring at a screen, it may be caused by the eye fatigue brought on by Computer Vision Syndrome. If this is a problem for you, here are some ways to tweak your behavior.

[h/t The Verge]

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Could You Pass the Mensa Intelligence Test?
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The biggest perk of being in Mensa just might be the bragging rights. Membership is reserved for society’s most elite intellectuals, and the only path to induction is to score in the 98th percentile or higher on their notorious IQ test. Think you have the smarts to join the top .0001 percent of thinkers? Before you apply for the real thing, flex your brain by answering a few sample questions from the test.

The practice questions shared on Popular Science cover the skills Mensa looks for in its members: verbal, spatial, and mathematical reasoning. To ace the test, you’ll need to be just as capable of recognizing obscure vocabulary as you are of doing complex math in your head.

The real Mensa IQ test is really two tests: a timed test with 50 questions and one with seven sections. (Scoring in the 98th percentile on either test qualifies you to join.) Answering all four of the questions posted to PopSci correctly doesn’t necessarily mean you’re intelligent enough for Mensa, but it should give you a confidence boost if you’re thinking about applying. If you do miss a question or two, maybe hold off on taking the official test until you’ve had more time to prepare: Scoring below the 98th percentile bars you from joining the exclusive club for life. You can take the practice test now by heading over to PopSci.

[h/t Popular Science]

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