Ответ:

Mr(C6H14) = 12*6 + 1*14=86

W(C) = Ar(C) *n / Mr(C6H14) *100% = 12*6 / 86  *100% =84%

W(H) = Ar(H) *n / Mr(C6H14)  *100% = 1*14 / 86  *100% = 16%