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?
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?
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.
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.
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).
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.
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.
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).