Number of neutrons present in 3.2 grams of methane is equal to (assume methane contains C-12  and  H-1  isotopes)

Number of fundamental particles in carbon = 6p + 6$\overline{\mathrm{e}}$ + 6n

Number of fundamental particles in hydrogen = 1p + $1\overline{\mathrm{e}}$

Number of fundamental particles in methane$\left({\mathrm{CH}}_4\right)$ = 10p + 10$\overline{\mathrm{e}}$ + 6n

$$\begin{gathered} \mathrm{GMW} \mathrm{of} {\mathrm{CH}}_4\left(16\mathrm{g}\right) \mathrm{contains} 6{\mathrm{N}}_{\mathrm{A}} \mathrm{neutrons} \\ 3.2\mathrm{g} \mathrm{contains} \text{'}\mathrm{X}\text{'} \mathrm{neutrons} \\ \mathrm{X}=\frac{3.2}{16}\times 6{\mathrm{N}}_{\mathrm{A}}=1.2{\mathrm{N}}_{\mathrm{A}}=7.2\times {10}^{23} \end{gathered}$$