let 'x' be the number of ounces of water to be added
@15% solution, water is 100%-15% = 85%
@5% solution, water is 100% - 5% = 95%
i did find for the amount of water in the solution since this involves the amount of water added not baking soda
you'll have the equation as:
the amount of water in ounce + amount of water to be added = resulting concentration
12(.85) +x(1) = 0.95(12+x)
10.2 + x = 11.4 + 0.95x
x - 0.95x = 11.4 - 10.2
0.05x = 1.2
x = 24 ounces