Republican vice presidential nominee Paul Ryan on Sunday insisted that GOP hopeful Mitt Romney had provided specifics for his tax plan, but refused to say which deductions would need to be eliminated or provide any math to prove that the scheme works.

During an interview on Fox News, host Chris Wallace noted that a recent study showed that the Romney-Ryan plan would cost nearly $5 trillion over 10 years.

"Not in the least bit true," Ryan insisted. "That study has been so thoroughly discredited."

"How much would it cost?" Wallace wondered. "The cut in tax rates."

"It's revenue neutral... Lower all Americans' tax rates by 20 percent," Ryan replied.

"Right, how much will it cost?" Wallace pressed. "It's not revenue neutral unless you take away the deductions."

"I won't get into a baseline argument with you because that's what a lot of this is about," Ryan explained. "We're saying, limited deductions so you can lower tax rates for everybody. Start with people at the higher end. ... And every time we've done this -- whether it was Ronald Reagan working with Tip O'Neil, the idea from the Bowles-Simpson commission on how to do this -- there's been a traditional Democrat and Republican consensus: lowering tax rates, broadening the tax base works."

"But you haven't given me the math," the Fox News host pressed.

"I don't have the time," Ryan laughed. "It would take me too long to go through all the math. But let me say it this way, you can lower tax rates by 20 percent across the board by closing loopholes and still have preferences for the middle class for things like charitable deductions, for home purchases, for health care. So what we're saying is, people are going to get lower tax rates."

"If -- just suppose -- that the doubters are right, President Romney takes office the math doesn't add up... what's most important to Romney?" Wallace asked. "Would he scale back on the 20 percent tax cut for the wealthy?"

"No," Ryan said.

"Would he scale back and say, 'OK, we're going to have to raise taxes for the middle class?'" Wallace continued. "What's most important to him in his tax reform plan?"

"Keeping tax rates down," the vice presidential candidate remarked. "That's more important than anything."

During an event in Ohio last week, President Barack Obama said that Romney and Ryan had refused to provide details because it was impossible for them to reduce the deficit and cut taxes for the wealthy without also raising taxes on the middle class.

"No matter how many times they try to reboot their campaign, no matter how many times they try to tell you they’re going to start talking specifics really soon, they don’t do it, and the reason is because the math doesn’t work," Obama asserted.

Watch this video from Fox News' Fox News Sunday, broadcast Sept. 30, 2012.