The exact molecular mass for butane (C4H10) is
12.0096*4+1.0079*10=58.1174 which is 58.1 to 3 significant figures.
Proportion of carbon in the compound
12.0096*4: 58.1174
=>
48.0384 : 58.1174
The mass of carbon in 2.50 grams of butane can be obtained by proportion, namely
Mass of carbon
= 2.50 * (48.0384/58.1174)
= 2.0664
= 2.07 g (approximated to 3 significant figures)