Maya’s Stamps Errand

Maya’s mother asked her to buy some stamps. The available stamps were 3 cents, 9 cents, 11 cents, 17 cents and 21 cents. Her mother asked her to buy eight each of three stamps and nine of each of the remaining two stamps. Unfortunately, Maya forgot which stamps she was supposed to buy eight and nine of. Luckily, her mother had given her $5, the exact amount required to buy the stamps. Which stamps did she buy?

Eight of the 11 cent, 17 cent and 21 cent stamps and nine of the 3 cent and 9 cent stamps.

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

Worn By Men For Privacy

I’m an argument of why my side should win,
And worn by men for privacy.
I don’t last long if I get under your skin,
And I rhyme with a kind of misery.

What am I?

Brief (or briefs). A legal brief is a short outline of the arguments in a court case, briefs are worn by men to cover their privates. Brief also means to not last long and brief rhymes with grief, a kind of misery.

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Winning at Wimbledon

As a result of temporary magical powers, you have made it to the Wimbledon finals and are playing Roger Federer for all the marbles. However, your powers cannot last the whole match. What score do you want it to be when they disappear, to maximize your chances of hanging on for a win?

It sounds obvious that you should ask to be ahead two sets to love (it takes 3 out of 5 sets to win
the men’s), and in the third set, ahead 5-0 in games and 40-love in the sixth game. (Probably you
want to be serving, but if your serve is like mine, you might prefer Roger to be serving the sixth
game down 0-40 so that you can pray for a double fault.)

Not so fast! These solutions give you essentially 3 chances to get lucky and win, but you can
get six chances—with three services by you and three by Roger. You still want to be up two sets
to none, but let the game score be 6-6 in the third set and 6-0—in your favor, of course—in the
tiebreaker.

Credit goes to Peter Winkler for creating this.

Posted in Brain Teasers

Kevlar, Fire Escapes and Wipers, Oh My!

What do computer compilers, kevlar, fire escapes and windshield wipers all have in common?

They were all invented by women.

A variation of this brain teaser includes laser printers (they were invented by Gary Starkweather) and bulletproof vests (invented by Casimir Zeglen using silk, though modern vests use Kevlar, so it’s halfway right).

Posted in Brain Teasers

Seconds in a Year

How many seconds are there in a year?

Hint: You don’t need a calculator

24. If you said 12 for January 2nd, February 2nd, etc that’s close, but you forgot about January 22nd, February 22nd and so on. If you are a math whiz and didn’t need a calculator to perform 60 x 60 x 24 x 365, then 31,536,000 works too. If you used 365.25 to account for leap year, then you are a human calculator, but even that’s not entirely accurate due to the leap second. And even accounting for that, it’s only an approximation that there are 365.2422 days in a year.

Posted in Brain Teasers

amid eerie nighed won spawn

What phrase do these words sound like when rearranged?

amid eerie nighed won spawn

“Won spawn amid nighed eerie” sounds like, “Once upon a midnight dreary”, from The Raven by Edgar Allan Poe.

Posted in Brain Teasers

Same Three Digits with Sum of 800

When Florence and Willie finished playing darts, they proudly announced that their 3-digit scores added up to exactly 800 points. Furthermore, each of their scores shared the same 3 digits, without any repeating digits.

What were their two scores?

364 and 436.

You could figure this out mathematically, but instead, I plugged in digits that added up to 10 for the first column, 9 for the second column (since you carry the 1) and 7 for the third column (again, because you’re carrying the 1).

Posted in Brain Teasers

Growing Tall But Never Fat

This on this.
That on that.
Growing tall,
but never fat.

What am I?

A stack.

By Sef Daystrom

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When I Point Up It’s Bright

When I point up, it’s bright.
When I point down, it’s dark.

What am I?

A light switch

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