Reasoning
By likely, we mean greater than 50% chance.
With one person there is a 0 percent chance that you'll have two people with the same birthday.
With two people the probability that they won't share a birthday is 364 ÷ 365. The probability that they will share a birthday is therefore 1 − (364 ÷ 365).
With three people the probability that they won't share a birthday is the same as for two people, times 363 ÷ 365. So the probability that three people will share a birthday is 1 − (364 ÷ 365) x (363 ÷ 365).
Notice that with each additional person added, the probability that they shares a birthday with one of the previous persons goes up, because there are fewer "free" days remaining.
We keep adding people until the %age is greater than 50%.
When we have 23 people the %age is 50.729%.
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Puzzle 3
Frankie the Squirrel can find one acorn in 15 minutes and Glenn the Squirrel can find one acorn in 12 minutes. As autumn approaches, they both work all day and all night.
However, Alex the Squirrel is eating their collection! It takes Alex 20 minutes to eat an acorn, and Alex eats all day and all night long (and starts on yesterday's acorns).
How many extra acorns will there be at the end of a 24-hour day?
Reasoning
Frankie finds 4 acorns every hour, Glenn finds 5 acorns every hour and Alex eats 3 acorns every hour, so the nett total per hour is 4 + 5 − 3 = 6.
