What is the percentage composition by mass of carbon in hexane, C6H14, to the nearest decimal point?

You do have a printed periodic table, right? You can get the information at www.chemicalelements.com/index.html but you really need your own printed copy to carry with you. It will be a primary reference for everything you do.

There is a diagonal line, B-C, Si-P, Ge-As, Sb-Te, and Po-At. Elements to the left of the line are metals, meaning they lose an electron easily, elements to the right of the line are non-metals, and elements on the line are semiconductors.

Your question involves a lot of arithmetic and you just have to get used to that. Everybody will expect you to understand this method perfectly.

C - 12.011 x 6 = 72.066
H - 1.00794 x 14 = 14.1116
-------------------- ADD
86.177 (Decimals after the third one are dropped.)

72.066/86.177 x 100% = 83.6254643% rounded to 83.6%