The service plan for edith's cell phone charges her $29.50 a month for 300 minutes and 10¢ a minute for each additional minute over the 300 minutes. if edith uses 400 minutes this month, how much money will she have to pay?

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