Tommy did not seem to understand subtraction, so the teacher tried to make it plain with the following example:
‘Now, suppose Billy had fifty pence,’ said the teacher.
‘Yes’m,’ said Tommy.
‘And you asked him for twenty-five.’
‘How much do you think Billy would have then?’
‘Fifty pence,’ said Tommy with a discouraged look
— Herbert V Prochnow & Herbert V. Prochnow Jr. in Jokes, Quotes & One Liners Volume 2