Help with Drug Calculation

I am in my third week of nursing school. We just started drug calculations this week. I'm doing a practice problem in my book that has me stumped. Either the book has a typo or I am missing some concepts. The problem is as follows:

Order: ciprofloxacin 0.1g IV q12h

Available: cipro 200mg/20mL

Set and Solution: secondary IV set with drop factor of 15 gtt/mL; 100 mL D5W

Instruction: dilute ciprofloxacin in 100 mL of D5W and infuse over 60 min

Question one wants to know how many mL per does should be given, which I set up as:

100 mg / 200mg x 20 mL= 10 mL per dose

Question two wants to know the IV flow rate in drops per minute. I set it up like this:

100 mL x 15 gtt/mL / 60 = 25 gtt/min

The book says the answer is 28 gtt/min. I cannot figure out how they got that number. They set the problem up like this:

10mL + 100mL x 15gtt/mL / 60 min = 100 x 15 / 60 = 27.5 or 28 gtt/min

I am so confused right now. Can anyone explain this or show me how I am doing this wrong? Thanks in advance.