Determine total number of neutrons in three isotopes of hydrogen

No. of. neutrons in ${ }_1^1\mathrm{H}=0$
No. of neutrons in ${ }_1^2\mathrm{H}=1$
No. of neutrons in ${ }_1^3\mathrm{H}=2$
$\therefore$ Total No. of neutrons = 3