Shutterstock/ktsdesign
#17 Semi-one numbers
Set by Colin Wright (with thanks to Adam Atkinson)
When you count up to 2, exactly half of the numbers contain the digit “1”. We can use the term “semi-one numbers” to describe numbers with this property.
The same is true of a number between 15 and 20. Which one?
What is the next semi-one number after that?
Could there be infinitely many such numbers?
Solution next week
#16 Order, order!
Solution
There are many ways to arrange 1-9 so each pair differs by 2 or 3. For example: 1, 3, 5, 2, 4, 7, 9, 6,…
