Sorting Numbers

 

 

1. Arrange the digits 1-9 in a line so that each pair of adjacent digits differs by either 2 or 3.
2. Arrange the digits 1-9 in a line so that each pair of adjacent digits sums to a prime number.
3. Now arrange the digits 1-9 in a line so that each pair of adjacent digits (when read as a two-digit number) appears in the times tables from 1 × 1 up to 9 × 9.

 

 

 

        Source: Katie Steckles, The New Scientist