Here is how I completed the problem:
I started by creating a ratio:

<em>Where 'x' is the desired amount of the 18% solution of sulfuric acid.
</em>From here, I solved for 'x':
<em>
</em>







∴You will need to add 1080ml of 18% Sulfuric Acid in order to obtain a 15% solution.
40 percent 3/5 equals 60, 5/5 equals 100, 100-60 equals 40 percent
I believe (a+b)^5 would be the answer