I remember reading some quick math formulas? For example, a dozen times a dozen: head to head, tail to tail, tail to tail. Others have been forgotten. Does anyone know?
make hay of
13× 18=
1× 1= 1 3+8= 1 1 3×8= 24 13× 18= 100+ 1 10+24=234
Head to head is 100, tail to tail is 100 and 10, and tail to tail is 10 and 1, so add them up.