Mitt Romney said he doesn't know if he's ever paid a tax rate lower than 13.9 percent -- the rate he paid in 2010 according to the one year of tax returns he has released thus far -- the presumptive nominee told ABC's David Muir in an interview Sunday, via ABC's Emily Friedman. 

Romney tells @DavidMuir he doesn't know if there was a year when he paid a tax rate lower than 13.9%. Says he'll have to go back and look.

— Emily Friedman (@EmilyABC) July 29, 2012