You are at a party and overhear a conversation between Lucy and her friend.
In the conversation, Lucy mentions she has a secret number that is less than 100.
She also confesses the following information:
“The number is uniquely describable by the answers to the following four questions:”
Q1) Is the number divisible by two?
Q2) Is the number divisible by three?
Q3) Is the number divisible by five?
Q4) Is the number divisible by seven?
I loved this puzzle. I first solved it a brute-force way, with Excel. Then I found increasingly elegant and logical solutions. Then I shared it with some friends: I love it! Go read the whole thing.