Actually, I DO know where we got that figure. It's what the minimum wage would be if it had been pegged to inflation in 1968, which is the last time on record that minimum wage would have met the poverty line.
The American worker has, in effect, been taking a pay cut for fifty years. When your pay doesn't keep up with inflation, that's the same as taking a paycut. So if it worked in 1968, we should go back to that, and when you do the math, it comes out to $15 an hour. Burger flippers and gas pump attendents used to make that much (relative to the value of currency in their day) and then people like you decided that was too much, that they deserved less - but it's what they were getting.