Verified answer

HNO3 - m(речовини)= 300g*0,126=37,8g

m(H2O)= 300-37,8= 262.2

M= 63g/mol

n= m/M=37.8/63.8=0.59 mol

NaCl - M= 23+35,5=58,5

m=n*M=3*58,5= 175.5

m(H2O)= 400-175.5=224,5

W=(175.5/400)*100%=43,9%