My grandmother worked in the billing department of a department store at the beginning of the 20th century, before adding machines were in use. Everything was calculated by hand. She showed me how she could add up a column of three-digit numbers all at once. She had her own way of adding up a column of single-digit numbers, basically by picking out combinations that added up to ten. She'd gather the 7+3s, the 6+4s, the 8+2s, etc. and keep track of the unmatched ones somehow.

It started me on the path of realizing that there's more than one way to solve a problem.