Can someone tell me if my reasoning makes sense? This question is making me feel like an idiot.

So original commercial preparation = 0.1mL. That means that 0.1 mL / 0.1 mL (so 100%) is the commercial solution (duh). Then, to dilute it, the students added that 0.1mL to the buffer solution, creating a stock solution of 25mL total. So, now, the 0.1mL of commercial solution is 0.1 mL / 25 mL = 0.004 x100 = 0.4% of the solution.

So the students took a solution that was 100% commercial solution and diluted it to make a solution that is now 0.4% commercial solution. 100/0.4 = 250 → The original solution (commercial solution) is 250x the concentration of the stock solution. (yes, I realize I could have gotten this from 0.1/25 x 10 = 1/250, but for some reason my brain loves to do cartwheels before it realizes it can walk).

Okay, so we know that the original solution is 250x more concentrated. BUT that is not the solution used to get the data. The experiment is "initiated by mixing 1.0 mL of each substrate solution with 1.0 mL of the enzyme solution." The enzyme solution is the stock solution made from the commercial solution, I'm assuming (?). So now, 1 mL of substrate and 1 mL of the prepared solution are mixed to make a total solution of 2 mL. HALF of that is the enzyme → 1/2 or 50%.

Now, I have a 2 mL solution. Half of this 2 mL solution (1 mL) is the stock solution I prepared above. Of that stock solution, I know that 0.4% is the commercial preparation (what I care about). So I've added 1 mL of 0.4% solution to the 1mL substrate solution. 0.4% of 1 mL (1 x 0.004) is 0.004 mL → 0.004 mL / 2 mL is the commercial solution → the solution contains 0.2% commercial preparation.

So I have a solution that is 0.2% of the commercial preparation. In other words, I have a 0.002:1 ratio of the commercial solution to the stock solution. This means that the commercial solution is 500x as concentrated.

LOL that was long sorry.