Question 896653
Assuming all measures based on MASS, let x = relative amount of mass of water to add.


{{{55*1/(1+x)=25}}}, assuming some 1 unit of the starting 55% nitric acid; and that x is in the same unit, but both here are ASSUMED to be MASS.
Solve for x.


Note:  For promoting safety, one must add the acid to the water; NOT the water to the acid!   Your question is potentially dangerous.