Problem Solving Maths Problem!!!?

Usually, Peter and Paula meet at a bus interchange where buses depart at equal intervals of time. Once Paula came 15 minutes later than Peter and Peter saw 6 buses depart. At another time, Paula came 26 minutes later than Peter and Peter sadly watched 8 buses depart. But on a further occasion, Peter came 43 minutes later than Paula. How many buses departed the interchange while Paula was waiting for Peter? Find all possible answers.