To find the solution,we can first the amount of nitric acid contained in his solution:
12Ă—20%
=2.4 ounces
We can then find the percentage of nitric acid in his solution after adding 8 ounces of water:
=2.4/(12+8) Ă—100%
=2.4/20 Ă—100%
=12/100 x100%
=12%
As 12%<15%, he does not need to add more water.
Hope it helps!