Q: Start with 100 coins: 10 stacks of 10 coins each.In 9 of the stacks, the coins all weigh 1 ounce but one stack has coins that weigh 2 ounces. Using only a digital scale and only one weighing, identify the stack that contains the 2-ounce coins.

A: Number the stacks from 1 to 10.
Then represent each stack with its number of coins so Stack 1 will have 1 coin and Stack 2 will have 2 coins, etc.
These 10 stacks if all the coins weighed the same would be 55 ounces
(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10).
The difference between the actual weight and 55 would tell us which stack had the 2-ounce coins .
For example. if stack 5 was the stack with the 2-ounce coins, the weight would have been 60 (55+5).

Q: At 5 o’clock, it takes 5 seconds to strike 5 chimes on a chime clock. Assuming the chime takes negligible time, how long will 12 chimes take at 12:00?

A: 13.75 seconds
The total time is 5 seconds and there are 4 intervals between the chimes so each interval is 1.25 seconds.

At 12:00, there are 11 intervals between the chimes, therefore the chimes will take 11 x 1.25 seconds = 13.75 seconds.

Many thanks to Reader’s Digest for this fun and yummy brain teaser. Here is the solution and the link:
https://www.rd.com/list/spot-the-difference/

There are 10 books to choose from on a reading list. Mr. Lee would like his students to read any 5. Tameka chooses 5 books randomly as does Nadeem. Find the probability there are exactly two books they both choose.

10 choose 5 = 10 C 5 = 252
There are 252 ways to choose 5 books out of 10.

Tameka needs to pick 2 books from Nadeem’s list and 3 that are not on his list.

There are  10 ways for Tameka to choose 2 of the 5 books on Nadeem’s list. 5C2
From the remaining  5  books that aren’t on Nadeem’s list, there are 10 ways for her to choose 3 of those using 5C3.

10 x 10 so there are 100 possible combinations for the 2 books out of the total 252. 100/252 can be simplified to 25/63.