We are looking for the total amount of the solution. We only know part of it, that there are 18 milliliters of the alcohol. We also know that the alcohol makes up only 60% of the solution.
To find the whole, we can set up a proportion using the information given.
60 / 100 <--- This is our percentage, which we were given.
18 / x <--- This is the part (alcohol - 60%) over the whole, which we don't know and which also corresponds to the 100.
Therefore, our proportion is as such:
60 / 100 = 18 / x
To solve, cross-multiply.
100 * 18 = 60 * x
1800 = 60x
x = 30 total milliliters of the solution
Hope this helps!