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

Odd Row Out

Which row is the odd one out?

A.KNQTWZ
B.BFJNRV
C.AFKPVZ
D.369121518
E.71115192327
F.131823283338

C is the odd row out.

In A two letters are skipped between each letter.
In B three letters are skipped between each letter.
In C four letters are skipped between each letter except V, which should be U.
The numbers in rows D, E and F follow the same pattern.

Posted in Brain Teasers

What Do You Serve?

What do you serve that a polite recipient would immediately send back?

A ball, when you’re playing tennis, volleyball, table tennis, etc.

Posted in Riddles

Wholly Unreal

Something wholly unreal, yet seems real to I
Think my friend, tell me where does it lie?

In your mind.

Posted in Riddles

Company, Congregation or Convocation

Some form a parliament,
Others a company, congregation or convocation,
The mob can see a murder, or clamor and wake,
A bouquet is presented when flushed.

What are we?

Birds. These are collective nouns for groups of birds.
parliament – owls
company – parrots
congregation – plovers
convocation – eagles
mob – emus
murder – crows clamor – rooks
wake – buzzards
bouquet – pheasants (when flushed)

Posted in Riddles

Word With All 26 Letters

What word contains all 26 letters?

Alphabet (unfortunately there aren’t any actual dictionary words with every letter in the alphabet, so this gimmicky, somewhat unsatisfying answer has to suffice).

Posted in Riddles

What Work is Never Finished?

What work can one never finish?

An autobiography. Unless you’re writing it in your death bed I suppose, but even then, how will you be able to record the time of death?

Posted in Riddles

Add An A But Sounds The Same

What word starts with the letter “I”, becomes another word by adding the letter “A” yet has the same pronunciation?

Isle, Aisle

Posted in Brain Teasers

Fill With Empty Hands

What do you fill with empty hands?

Gloves.

Posted in Riddles

What is the Five Digit Number?

What 5-digit number satisfies the following requirements?

1. No zeroes
2. First two digits are the same
3. Fourth digit is twice the first
4. Last digit is twice the third
5. Sum of all digits is 18

33264.

There are several ways of solving this. One way is to represent each spot with a letter, A – E.

Then you have these equations:

A = B
D = 2 * A
E = 2 * C
A + B + C + D + E = 18

By simplifying, you get:

4 * A + 3 * C = 18

Plug in values for A and C to find that A = 3 C = 2.

Posted in Brain Teasers