Ben is a swimming instructor. Sue has paid Ben for 20 weekly group swimming classes. Sue tells Ben she wishes to enter a swimming competition in a month's time and she asks Ben if he can stay longer to give her extra one to one tuition. She offers to pay him an extra $60.

For the next month Ben stays on longer to help Sue. Ben asks Sue for an additional $60 for the extra one to one tuition. Sue refuses to pay him as she has already paid for the group classes.

Explain to Ben whether or not he is entitled to the additional $60. (plz attach more explanation)