Joan works on commission for a furniture store. She gets a base pay of $70 a day plus 4 percent of the value of any merchandise she sells that exceeds a total of $400. Joan wrote the expression 400 + 0.04(t − 70), where t represents her sales total in dollars, to compute the total amount of money that she will make. For sales of $575 Joan calculated that she will make $420.20. Explain Joan's error by completing the correct expression and how much she will make.