Larry Summers, a former top economic adviser to President Barack Obama who helped craft the government's response to the economic crisis, says that Mitt Romney's budget plan doesn't square with reality.

Romney said at the third presidential debate on Monday that his budget plan would bring "a balanced budget within eight to 10 years."

But Summers, a Harvard economist, told The Huffington Post on Tuesday that Romney's budget math doesn't add up. "This is alchemy," he said.

"Lead cannot be turned into gold. Two plus two cannot equal five," Summers said. "And 20 percent across-the-board tax cuts cannot be squared with balanced budgets without raising middle-class taxes or eviscerating government."