Cranmer Abacus–learn the rules to subtract by 2 – YouTube
https://www.youtube.com/watch?v=DVrUVEDZRi4
https://www.youtube.com/watch?v=DVrUVEDZRi4
https://www.youtube.com/watch?v=6x4A6Y2Lrps
https://www.youtube.com/watch?v=F917QtOuq3U
https://www.youtube.com/watch?v=gUUQhaRYetc
https://www.youtube.com/watch?v=MyALwnBQPjk
https://www.youtube.com/watch?v=R_Dpnf8NmJI
https://www.youtube.com/watch?v=3incVeysmZU
https://www.youtube.com/watch?v=rZKgj0YvjnE
https://www.youtube.com/watch?v=T_5kOpe4NIw
https://www.youtube.com/watch?v=9F-7nVr4BLw