How To Make One Disappear?

How can you add one item to the number one to make it disappear?

Add the letter ‘g’ to make it gone.

Posted in Brain Teasers

Largest Product

Using all the digits from 1 to 7, what two numbers have the largest product? For example, 1234 x 567 = 699678 but you can do much better than that.

742 x 6531 = 4,846,002.

A similar problem can be found in L.A. Graham’s Ingenious Mathematical Problems and Methods with a range of 1 to 9, but the principle remains the same – the numbers with the smallest difference produce the largest product. You start out with the highest two digits, 7 and 6, then attach 5 and 4, putting the smaller of the two digits with the larger number, giving you 74 and 65. The next two highest digits are 3 and 2, giving you 742 and 653. Finally, you add the 1 to the lower number. Page 80 has the details of that solution.

Posted in Brain Teasers

I Turn Around Once

I turn around once,
What is out will not get in.
I turn around again.
What is in will not get out.

What am I?

A key.

Posted in Riddles
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Something To Everybody And Nothing To Everyone Else

What is something to everybody and nothing to everyone else?

Your mind.

Posted in Riddles

Archaeologist Laughed At

An archaeologist publicizes his find, a coin marked 649 BC, but is laughed at by his peers. Why?

No coins were ever marked BC because the BC / AD dating system wasn’t even devised until 525 AD. And it wasn’t widely used until after 800 AD.

Posted in Brain Teasers

What is the Chance That You’ll Be Correct?

If you choose an answer to this question at random, what is the chance that you will be correct?

a) 25%
b) 50%
c) 60%
d) 25%

This is becomes a self-referential paradox. Both A and D would be correct if there were four unique answers, but since A and D are the same answer, the chance that you would choose a correct answer is 50%, which makes B correct. But if there’s only one correct answer, the odds of choosing the correct one at random goes back to 25%. And around and round you go.

There’s a lot of discussion at Richard Wiseman’s blog and more at Lifehacker, where I first saw this.

Posted in Brain Teasers

The Train and the Tunnel

A train is a half mile long and is about to enter a 10 mile tunnel. If the train is traveling at 35mph, how long will it take for the entire train to make it through the tunnel, from the front of the train entering to the end of the train leaving the tunnel?

18 minutes.

Length of the train: 0.5 miles
Length of the tunnel: 10 miles
Total distance: 0.5 + 10 = 10.5 miles

Time = 10.5 miles / 35 mph = 0.3 hours = 18 minutes

Posted in Brain Teasers

The Websters’ Busy Day

Mr. and Mrs. Webster were on a holiday shopping excursion. On their way to the car after leaving the store, Mr. Webster complained he was tired from carrying so many packages. Mrs. Webster, the kindly thing that she was, replied, “Quit your complaining Walter! If you gave me just one of your packages, I’d have twice as many as you. And if I gave you one of mine, we’d have the same.”

How many packages were Mr. and Mrs. Webster each carrying?

Mrs. Webster was carrying 7 packages and poor old Mr. Webster was laden with 5.

Posted in Brain Teasers

Eight Eleven Five Sequence

Which two numbers are missing and where do they go in the sequence?

8, 11, 5, 14, 1, 7, 6, 10, 13, 3, 12, 2

The missing numbers are 4 and 9. The list is sorted alphabetically by the English spelling of the numbers, so four belongs after five and nine comes after fourteen.

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You Read Me Daily

I am a seven letter word, you read me daily. Letters 567 increase every year. The third and fourth letters are the same and my 325 letters cover about 70% of the world. What am I?

message. Letters 567 are your age, which goes up every year. Letters 3 and 4 (both s) are in fact the same, and letters 325 spell sea, and estimates say that the earth is covered by about 70% water.

Posted in Riddles