I need my helper to work on a Sunday in a few week's time. We've never had to do this before, but we need to attend to some business overseas over the weekend and need her to look after our baby. I've kindly asked her and she said it was OK. I told her that she can substitute that Sunday for another day when we return, however, she said that she would prefer to be paid extra instead. How much should we pay her for working on that Sunday?