Rotating Coins Around Coins

You have two coins of equal size on a table, situated end to end vertically. If you roll the top coin around the bottom coin until it returns to its original position, how many times will it have rotated?

Twice. See this video for more details.

Posted in Brain Teasers

Marble Walls White as Milk

In the marble walls as white as milk,
Lined with skin as soft as silk,
Within a fountain crystal clear,
A golden apple does appear.
No doors are there to this stronghold
Yet thieves break in and steal the gold.

An egg or egg yolk. This is a variation of A Box Without Hinges.

Posted in Riddles

Three Kinds of Apples

Three kinds of apples are mixed randomly in a box. How many apples must you take out to be sure of having at least two apples of one kind?

Four. If the apples are A, B, and C, it’s possible that you could take out one A, one B and one C with three apples, but the fourth apple must be one of the three kinds.

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

B C D

B, C, D, E, G, P

What is the next letter in the sequence?

T. All of the letters rhyme.

Posted in Brain Teasers

A Seemingly Simple Math Problem

What is the answer to this problem?

1 + 1 + 1 + 1 + 1
1 + 1 + 1 + 1 + 1
1 + 1 × 0 + 1 = ?

30.

There are two tricks to this one. The first is 1 × 0. But it’s mostly there as a distraction. Yes, 1 × 0 is 0, but that doesn’t affect anything else in the equation. The second, far trickier element is that the lines ending with 1 don’t have a + sign next to them. That means they should be combined with the following line.

Let’s put the equation all on one line, shall we?

1 + 1 + 1 + 1 + 11 + 1 + 1 + 1 + 11 + 1 × 0 + 1 = ?

And the answer to that equation is 30.

Posted in Brain Teasers

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

Amber Knits and Fauna Worry

Find a single letter that replaces the first letter in the two words of each row to make two new words. What word is formed by combining the new letter of each row?

For example: PLANK _ BRISK

The new letter is F, making FLANK and FRISK. And F would be the first letter of the answer.

AMBER _ KNITS
FAUNA _ WORRY
WARNS _ AERIE
FEMUR _ GAILY

1. U – UMBER and UNITS
2. S – SAUNA and SORRY
3. E – EARNS and EERIE
4. D – DEMUR and DAILY

The new word is USED.

Posted in Brain Teasers

The Letters of Man

Using only the letters in MAN to fill in the blanks, make normal, uncapitalized words.

1. _ _ O _ G
2. R _ _ D O _
3. _ _ R I _ _
4. E _ P _ _ _ D _
5. C I _ _ _ _ O _
6. P _ _ O R _ _ _
7. _ _ I _ L _ _ D
8. _ R _ _ _ E _ T

1. AMONG
2. RANDOM
3. MARINA
4. EMPANADA
5. CINNAMON
6. PANORAMA
7. MAINLAND
8. ARMAMENT

Posted in Brain Teasers

Noah, Emma and the Ice Cream Truck

Noah and Emma, track stars with superb hearing, ran as fast as they could to catch up to the ice cream truck in the next town over. They averaged 6 miles per hour, then rested to enjoy their ice cream. They averaged 4 miles per hour over the same distance on the way back. Not counting the time they rested, what was their average speed?

4.8 miles per hour. 5 mph is tempting, but incorrect. Pick any distance, say 12 miles there and 12 miles back. They take two hours to cover the first 12 miles and three hours to cover the return trip. In 5 hours they covered 24 miles which is an overall average of 4.8 miles per hour.

Posted in Brain Teasers