6. If earplug and earmuff are used together, will the NRR be total of both the equipment’s ?

Earplugs worm in combination with earmuffs, helmets or communications headsets, typically provide greater protection than either device alone. However, the attenuation of the combination is not equal to the sum of the individual attenuation values For example, at 1000 Hz the combination of a 19 dB plug and a 21 dB muff does not yield 40 dB overall, but rather about 31dB. The simplified solution would be to add 10 dB over the highest device viz 21 dB+10 dB = 31 dB.