There was some question about # 25 on page 106. The answer in the back of the book states 30 mL. If you use the conversion on the chart, it states 5 mL = 1 tsp = 1 dram. According to that conversion, the answer should be 40 mL. However, at the bottom of the chart, it explains that the dram actually is worth 3.7 mL and has been rounded to 4 mL. Additionally on the chart, under 30 mL, it shows: 30 mL = 2 tbsp = 1 oz = 8 drams. Therefore, because it states "8 drams" that would be the most accurate conversion. If you used 3.7 as your conversion factor, your answer would be 29.6, which rounds to 30 as well.
I know this can be very confusing. I would like to apologize for it, but it is not my fault. Needless to say, there is a reason Harry Potter hated Snape and why simply following the potions formulas would not produce consistent results from student to student. That is why when Harry got a copy of the Half-Blood Prince's potion book, the notes provided the accurate amounts. Finally Harry was achieving the desired results while everyone else struggled, including Hermione. I think, most likely, the measurements were given in drams. So at least you are good company.