By "back then" I was thinking of about a century ago, i.e. WW1. Children spent 7 years at school having a basic education drummed into them. Even further back, in the previous century, long before compulsory education, elementary schooling was available for children at day schools and night schools for adults. Provision was patchy and basic. Attendance of many pupils was brief and irregular, but some of them would have learned something. Some schools were set up by employers. The owner of a coal mine in SW Scotland in 1840 only employed boys who could complete the application form in their own handwriting.
Miners at some Scottish pits paid a compulsory weekly school fee. If a miner had no child of his own to educate he could send a neighbour's child to school.
Working people were paid wages, they handled money, they managed household budgets, all that requires some arithmetic. Just because some of them couldn't use a quill pen to write their names it doesn't mean they were ignorant.
