The relative mass of a carbon atom is #12"gmol"^-1#, or #12# grams per mole.
The question tells us this is #10%# of the total mass of the molecule, so we can calculate this simply by
#12"gmol"^-1 " * ((100%)/(10%)) = 12 * 10#
#= 120"gmol"^-1#
This is the minimum mass of the compound, because if you have, say, a mass of #110"gmol"^-1# then carbon, which is #10%# of the total, comes to only #11"gmol"^-1#, though we know that carbon-12 has to come to #12"gmol"^-1#, by the very definition of carbon-12.
This is only a minimum mass, though, because you could have, say, a mass of #240"gmol"^-1# in which case #24"gmol"^-1# of it is carbon, so you have #2# carbon atoms (#24/12 = 2#), even though the total carbon percentage is still #10%#. So you can have more mass than #120"gmol"^-1#, but not less.
