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Before electronic calculators were invented in the 1960s, calculations were done in the head, on paper, with slide rules or on mechanical machines (sometimes powered by electricity to drive the mechanism).
Many people were very proficient at adding up complex numbers such as pounds, shillings and pence (£-s-d) in their heads. It was very difficult to devise a calculator to add up £-s-d so Britain changed to decimal currency in 1971.
£     s     d
5     16   3
2     5     8
3     14   6
11   16   5 
1     1