Although I'm an advocate for formulas, I am writing this in case you can't remember one for a test or quiz. Remember that dosages always uses simple math, always. There are no tricks and it can usually be answered without formulas if you forget them. So, if I forgot which formula to use I could still answer this question.
I would first start by figuring out how much time I'm infusing. That is the easy one. 0800 to 1500 is 7 hours. Subtract one hour for a break and you infuse for 6 hours total (or 360 min).
Next, I'd figure how many total drops fall during that period of 360 min. So, if 25 drops fall every minute then multiply 25 times 360 min which equals 9000 drops. Simple so far and no need to sweat.
Finally, it is important to understand what a drop factor is to keep going. It simply means that 15 drops equals 1 mL. It was given to you as 15drops/mL. So, if you divide 9000 drops by 15drops/mL you are left with 600mL.
Once again, I'm not advocating that you do this. My point is that this is simple math that can be used without any formulas if you forget them. In fact, the formulas really just do what I did above in a clear and concise way (I could argue that some formulas used are harder than what I just wrote.) If I have time, I check my formulas vs. simple math just to verify I chose the right formula.