In first $2 \mathrm{s}$, number of nucleons present is $900$ out of $1000$,
This means Percentage number of nuclei left $=\frac{900}{1000}\times 100=90\mathrm{\%}$, i.e., $90\mathrm{\%}$ nuclei are left behind.
Therefore in next $2 \mathrm{s}$, $90\mathrm{\%}$ of $900$ nuclei will be left.
Number of nuclei left $=\frac{90}{100}\times 900=810$, i.e., $810$ nuclei will be left.