Need help with reconstitution question

Hi everyone,

I'm a nursing student and new to this site. I have a math question that I cannot solve. I have the answer supplied by the instructor, but I don't know how to solve it. Could someone please walk me through it.

The question states:

Order: Clindamycin 300 mg, IV, q8h

Drug Available:

Tubing set and solution: 100 mL of D5W and a set with a 20 gtt/mL drop factor.

Instructions: Dilute in 100 mL D5W; infuse over 20 min.

How many of Clindamycin should be added to the 100 mL D5W IV PB solution?

I e-mailed the insturctor. Her reply didn't help me solve it...This is a reconstitution question, ignore everything except what it is asking for which is " How many mL of Clindamycin should be added to the 100 mL D5W IV PB solution?" You have 600 mg/4mL's. You need 300 mg. Divide 600 by 300 = 2 mL need to be added to the solution to get 300 Clindamycin. The answer is 2 mL.

I don't know how she got the 600/4mL. Can someone please help. Thank you!