Get Free Riddles and Brain Teasers Every Month

The Potato Paradox

Fred brings home 100 pounds of potatoes, which (being purely mathematical potatoes) consist of 99 percent water. He then leaves them outside overnight so that they consist of 98 percent water. What is their new weight?

50 pounds.

100 lb of potatoes with 99% water weight means there’s 99 lb of water and 1 lb of solids, a 1:99 ratio.

If the water decreases to 98%, then the solids account for 2% of the weight. The 2:98 ratio reduces to 1:49. Since the solids still weigh 1 lb, the water must weigh 49 lb for a total of 50 lbs for the answer.

Read more at the Potato Paradox wikipedia page.

Posted in Brain Teasers

Important Rebus

What does this represent?

IMPORTANT = IMPORTANT

Equally important.

Posted in Riddles
Tagged with

An Ancient Invention

What ancient invention allows people to see through walls?

A window.

Posted in Riddles

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

Paula and Quincy

Paula had four times the money that Quincy had, but after Paula gave Quincy $33, Quincy had 3 times as much as Paula.

How much did they each start with?

Paula had $48 and Quincy had $12.

If Q is the amount Quincy had, then 3(4Q – 33) = Q + 33

Solved, Q = 12.

Posted in Brain Teasers

How to Save Gas

If you could chose to increase the average fuel economy of a car, which of these would save the most fuel?

1. 10mpg -> 12mpg
2. 12mpg -> 15mpg
3. 15mpg -> 20mpg
4. 20mpg -> 30mpg
5. 30mpg -> 60mpg

It turns out they all save the same amount of fuel.

If you travel 100 miles:

100 / 10 = 10 gallons used
100 / 12 = 8 1/3 gallons used

10 – 8 1/3 = 1 2/3 gallons saved

100 / 30 = 3 1/3 gallons used
100 / 60 = 1 2/3 gallons used

3 1/3 – 1 2/3 = 1 2/3 gallons saved

Posted in Brain Teasers

All Words From Nine Letters to One

What 9-letter word can have a letter removed to make a new word and repeat the process until a single letter remains?

startling
starting
staring
string
sting
sing
sin
in
i

Posted in Brain Teasers

You’re Halfway There

You’re standing 11 feet away from a doorway and with each step you move halfway to the doorway. How many steps will it take for you to get to the doorway?

Technically, the answer is infinity. If you keep going halfway, you’ll never actually make it to the doorway.

But in practice, after 10 steps you’ll be 1/8th of an inch to the doorway and anyone behind you would say to stop being a baby and take a real step through the doorway already.

Posted in Brain Teasers

Its Own Past Tense

What common verb becomes its own past tense by rearranging its letters?

Eat (ate).

Posted in Riddles

None There’ll Be

There is a word of letters three,
Add one letter to it and none there’ll be.

What is the word?

One. Add an ‘n’ to get none.

Posted in Riddles