In the summer of 1976, just before my freshman year at the University of California at Berkeley, I worked as a groundskeeper at a neighborhood swim club. The regular minimum wage was $2.20 per hour, but the club was so cheap that I got paid the federal minimum wage for farmworkers. (I did a lot of weeding and watering.) So, I earned just $2 an hour.

But that was more than enough to pay my entire UCB tuition and fees. With one summer of full-time work, I or any real farmworker could make $960, about 50% more than the $637.50 for a whole academic year's tuition and fees at one of the best universities in the world.

Today, I teach at UCB, and I watch many of my students struggle with crushing debt. And no wonder. California just raised its minimum wage to $9, but at that pay, 12 weeks of summer work covers only a third of today's $12,972 tuition and fees.

Even if the movement for a $15 minimum wage for fast-food workers succeeds, a summer's work would cover slightly more than half of the tuition and fees for the 2014–2015 academic year.

One of the main reasons that state universities exist is to open up college education — and the opportunities that come with it — to economically disadvantaged students. So, how much college a summer minimum-wage job buys seems a reasonable measure of how accessible college education is. In different ways, the following charts show exactly that, from the 1970s to the present.