How much teenagers must borrow for college often depends on whether their parents can help foot the bill – and how much they can afford. Fresh data from a survey by Sallie Mae, the private college lender, shed light on how low-, middle- and high-income families find the money to pay for a college education. The data break down how much of students’ total costs – tuition, plus books, room and board, fees, living expenses, and transportation – come from earnings, savings, borrowing, grants or other sources. Here’s what stands out in the data, which are displayed in this chart: In low-income families, the students themselves take responsibility for saving, earning or borrowing money to cover 32 percent of their…