All posts by mathconfidence

January 2022 Brain Puzzler Solution

Q: A snail is at the bottom of a 12-foot well. She climbs up 3 feet during the day, but slides back 2 feet each night. How many days will it take her to get out of the well?

A: 10 days
After 5 days, she is 5 feet from the bottom. After 6 days, she is 6 feet from the bottom. It will take her less than 12 days.
At the end of the 9th day, she will be 9 feet from the bottom so on the 10th day when she climbs up 3 feet, she will reach the top!

December 2021 Brain Puzzler Solution

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).