Volume of H₂O added = 175 ml
Further explanation
Given
100 gm of a 55% (M/M) and 20% (M/M) nitric acid solution
Required
waters added
Solution
starting solution
mass H₂O = 45%=45 g
%mass of H₂O in new solution = 100%-20%=80%
Can be formulated for %mass H₂O :

For water mass=volume(density = 1 g/ml)
So volume added = 175 ml