What will be the resultant of two noise levels of 81 dB and 90 dB generated by two independent sources?

Sound levels in decibels combine on a logarithmic scale, not by simple addition: $L=10\log_{10}(10^{8.1}+10^{9.0})$. This is $10\log_{10}(1.126\times10^{9})\approx90.5$ dB, about 91 dB. Because 81 dB is 9 dB below 90 dB, it carries only about one eighth of the energy and adds only about half a decibel, so results such as 96 dB or 118 dB are far too high.