Medicare's Math Problem: Taxes - Benefits = Trouble

An average 66-year-old couple kicks in about $100,000 in Medicare taxes over the course of their lives. They've paid in, so they expect Medicare to pay out. ? and it does, to the tune of $300,000, on average. So how did the system become so unbalanced?

Source: http://www.npr.org/2011/04/30/135844222/medicares-math-problem-taxes-benefits-trouble?ft=1&f=1014

nancy pelosi harry reid john mccain al gore bill clinton