False Positive HIV Test

Imagine an HIV test that is 95% accurate (false positive rate of 5%) and around 2% of the tested population is infected with HIV. What is the probability that you actually have HIV when your test comes back positive?

29%

To read more about this, see the False Positive Paradox page on Wikipedia.

Posted in Brain Teasers

What’s The Captain’s Name?

I was standing by a railing,
watching a ship a sailing.
What is the Captain’s name.
If you don’t know his name,
It’s you to blame.
What is the Captain’s name.

“What” is the name of the Captain.

Thanks to Sherry for submitting this one. Here’s the backstory of the riddle.

“One of my favorites I remember as a child from my father, he always said it was a Newfoundland riddle. I don’t know where it may have originated. This is usually spoken. Ensure you say it as a statement and not a question.”

Posted in Riddles

When Is The Equation Correct?

When is the following equation correct?

14 + 1 = 3

When dealing with 24-hour time, 1400 hours plus 1 hour is 1500 hours, also known as 3 o’clock.

Posted in Brain Teasers

I Am And Yet Cannot

I am and yet cannot,
I’m an idea, yet can rot,
I’m two but none,
On land but on sea.

What am I?

A paradox / pair of docks.

Posted in Riddles

Has Teeth But Can’t Bite

What has teeth but can’t bite?

A comb. Other inanimate objects with teeth like a saw, zipper or a gear can “bite” you. Many people can affirm from painful experience in regards to zippers and saws, and the unfortunate factory worker who is missing a finger can attest to gears. It’s a rare event that anyone would be injured by a comb.

Another grim option is a dead animal or person.

Posted in Riddles
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Nine Matchbox Sticks To Ten

I have nine matchbox sticks and would like to make ten. How do I do it?

Arrange the nine matchbox sticks like so to make the shape of the number ten:

        _
    /| | |
     | |_|
Posted in Brain Teasers

My Thunder, Lightning and Rain

My thunder comes before the lightning.
My lightning comes before the clouds.
My rain dries all the land it touches.

What am I?

A volcano.

(via Reddit)

Posted in Riddles
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Sharpshooter

A sharpshooter hangs up his hat, turns around and walks 5000 meters in one direction, then turns around and shoots his gun, putting a hole right through his hat. How did he do it?

He hung the hat up on his gun.

Posted in Brain Teasers

Hello Father

A women and daughter walked into a restaurant. A man walked past and the women both said, “Hello, Father”. How is this possible?

The man was a priest.

Posted in Brain Teasers

One Strand Dangles

One strand dangles.
Two strands twist.
Three or more can
fashion this.

Braids

By Sef Daystrom

Posted in Riddles