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

A Long Bicycle Ride

Hester goes out for an afternoon bicycle ride. She rides for one hour at five miles an hour, then three hours at four miles an hour and finally two hours at seven miles an hour. How many miles did she ride in total?

31 miles.

1 hour at 5 mph = 5 miles
3 hours at 4 mph = 12 miles
2 hours at 7 mph = 14 miles

5 + 12 + 14 = 31 miles

Posted in Brain Teasers

Who May Enter?

A green glass door admits only certain objects. Apples and balls are allowed, but pears and bats aren’t. What determines whether an item can enter?

All words with a repeated letter are allowed. Green glass door, apple and ball.

Posted in Brain Teasers

The Longest Pregnancy

A baby was born four years after its parents died in a car crash. How was this possible?

Amazingly, this actually happened. The parents died in a crash crash and their parents fought for the rights to the four frozen embryos left by their late children. After a surrogate pregnancy, the baby was born with DNA matching their deceased parents.

Posted in Brain Teasers

Wait 27 Days Or See It Every Day?

You could wait 27 days or you could see it everyday. What is it?

The moon. Even though the true moon cycle is about 29.5 days, it takes 27 days, 7 hours, and 43 minutes for our Moon to complete one full orbit around Earth. Thus you can see the full moon every 27 days or you can moon yourself every day in the mirror. Not that you’d want to. Does anyone moon themselves? Can’t say as I’ve ever had the “pleasure”.

Posted in Riddles

All Those Legs

Two legs sat upon three legs with one leg in his lap.
In comes four legs, grabs one leg, and runs off with him.
Up jumps two legs, grabs up three legs, throws it after four legs,
and makes him bring back one leg.

Who are we?

One leg is a leg of mutton. Two legs is a person. Three legs is a stool. Four legs is a dog.

Posted in Riddles

A Surprise Party

What is a surprise party without any cause for celebration?

An ambush.

Posted in Riddles

Special Two Letter Combinations

What do these three letter combinations have in common?

DE
HI
MN

Hint: These combinations don’t have it in common.

AK
IN
WY

They are the only three US state abbreviations whose two letters are next to each other in the alphabet.

DE – Delaware
HI – Hawaii
MN – Minnesota

Posted in Brain Teasers

I’m Not an Enjoyable Friend

I am something many people don’t enjoy having as a friend, including you. But I am called upon anytime someone is injured. I have 5 letters and when my last letter is put before my first letter, I become a country.

What am I?

Pains. No one enjoys being in pain, when you’re injured you have pain, and when you move the s to the front, it becomes Spain.

Posted in Riddles

Ex-Spouse’s Former Daughter-In-Law

What do you call your ex-spouse’s former daughter-in-law’s first husband’s daughter?

You can call them whatever you want, but they have no familial relationship to you.

Posted in Brain Teasers