The relative mass of a carbon atom is 12gmol−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
12gmol−1⋅(100%10%)=12⋅10
=120gmol−1
This is the minimum mass of the compound, because if you have, say, a mass of 110gmol−1 then carbon, which is 10% of the total, comes to only 11gmol−1, though we know that carbon-12 has to come to 12gmol−1, by the very definition of carbon-12.
This is only a minimum mass, though, because you could have, say, a mass of 240gmol−1 in which case 24gmol−1 of it is carbon, so you have 2 carbon atoms (2412=2), even though the total carbon percentage is still 10%. So you can have more mass than 120gmol−1, but not less.
