Why does plus change to minus in 1st-year junior high math 'transposition'? Equations seen through a balance scale

This page is about 'transposition' (iko), which is taught in the first year of junior high school mathematics.Transposition ...
The Chronicle of Higher Education

Casting Out Nines: Math Monday: Columnar transposition ciphers and permutations, oh my

I hope you enjoyed Ed’s guest posts on NP-complete problems on TV the last couple of Mondays. It’s always great to hear from others on math that they are thinking about. This week it’s me again, and ...