Part of the reason college is so expensive now is that tuition has risen much faster than inflation. But the other reason is that wages have stagnated.

The result is that working your way through college, possible in the 1970s and '80s, is now a thing of the past.

At the flagship public universities in most states, students working 20 hours a week at minimum wage would have to work for more than a year in order to afford a year's worth of tuition, as this map from the Chronicle of Higher Education's Sandhya Kambhampati and Meredith Myers shows:

Flagship universities tend to be the most prestigious and best-known, but also the most expensive. Still, this map is just about tuition. It doesn't even count room and board or living expenses, which at some public universities exceeds tuition and fees.

The Chronicle has built a fascinating tool that lets you test different minimum wage levels and see how much tuition a year's worth of earnings at 20 hours per week would cover. In some states, even raising the minimum wage to $15 wouldn't be enough.

It's a good reminder that increasing tuition prices aren't the only reason college can feel unaffordable. Stagnating wages mean students can't contribute as much as they used to to tuition bills, either.