I know my father well

A math teacher  asked, 'Joey, if your father borrowed $300 and promised to pay back $15 a week, how much would he owe at the end of then week?'
'Three hundreds dollars,'  the boy  replied quickly.
'I'm afraid you didn't know your lesson very well,' the teacher scolded.
'Well,' Joey replied. 'I'm afraid you don't know my father.'