What is the mass percent concentration of a NaOH solution formed by mixing

The density of what is 2.45? Of the initial solution? And 2.45 grams/ml?
And the 25% is w/w? or something else?
If it's w/w, then you just find the mass of the initial solution: 50*2,45=122,5g.
The 25% of the mass is NaOH: That's 30,625g.
At your new solution, you have 10ml extra water. That's 10g extra water, since the density of water is 1g/ml.
But, the weight of NaOH remains the same. That means, the percentage of NaOH mass should be:
30,625/(122,5+10) <=>30,625/132,5 which is around 23,1%.


If it's not like that, please reply back to clarify.

At your future questions please be more specific. Always include all information, even if you think they are obvious.