So, \(420 gm CO _{2} \Rightarrow \frac{12}{44} \times 420\)

\(\Rightarrow \frac{1260}{11} gm\) carbon

\(\Rightarrow 114.545\) gram carbon

So, \(\%\) of carbon \(=\frac{114.545}{750} \times 100\)

\(=15.3 \%\)

\(18 gm H _{2} O \Rightarrow 2 gm H _{2}\)

\(210 gm \Rightarrow \frac{2}{18} \times 210\)

\(=23.33 gm H _{2}\)

\(So , \% H _{2} \Rightarrow \frac{23.33}{750} \times 100=3.11 \%\)

\(\approx 3 \%\)