A carbon atom has a relative atomic mass of 12.8. 60% of the carbon atom has a mass number of 12.7, 30% of the carbon atom has a mass number of 13.8, and the remaining 10% has a mass number that is not known. Calculate the mass number of the carbon isotope which is 10% of the carbon 12.8 atom. Give your answer to 1 decimal place, if needed.