Explanation

If the phrasing of these solution questions ever confuses you, remember that they are just ratio questions, and that means we'll write fractions with unit labels. This solution can be written as follows:



Most people with a sense of efficiency wouldn't write the units so fully on their noteboards, but it's good to write something. We need to change the bottom of this ratio to 40 without changing the ratio, so we will multiply by 1:







Then, . We can do a sanity check: the solution has dropped by more than half. So has our amount of active ingredient.

The correct answer is (E).