Consider the following reactions A →[ (ii) H_3 O^+] (i) CH_3 MgBr B →[573 K ] Cu 2-methyl-2-butene The mass percentage of carbon in A is \ \ \ \.

Consider the following reactions $$\mathrm{A} \xrightarrow[\text { (ii) } \mathrm{H}_3 \mathrm{O}^{+}]{\text {(i) } \mathrm{CH}_3 \mathrm{MgBr}} \mathrm{~B} \xrightarrow[573 \mathrm{~K}]{\mathrm{Cu}} \text { 2-methyl-2-butene } $$ The mass percentage of carbon in A is \_\_\_\_.