No matter how much or little she talks, she'll pay at least $29.50 a month. This is for 300 minutes, meaning she's 100 minutes over the maximum (400 - 300). We then multiply this amount by 0.10 and add it to 29.50. To write this as an equation, it would be 29.50 + x0.10 where x is the number of minutes over 300. In this case 29.50 + (100)0.10 = 29.50 + 10 = $39.50
