Laurie,
Whatever the reason is for the patient being given the IV drugs, is going to be what you use for the initial IV administration.
So, if the patient is coming for therapeutic infusion, along with an IV push, and it is not chemotherapy, the first code for administration would be 90765, if the IV lasts longer than 60 minutes, then also look at 90766 for each additional hour after the first hour. In order to use 90766, the infusion must go beyond 90 minutes total.
You are correct in going to 90775 for the IV push as the second administration, as long as the medication being pushed is given after the infusion (sequential) and not along with (concurrent) the infusion.
As you may see, documentation of start and stop times of infusions and pushes will be vital to being able to code drug administration correctly.
Lisa is correct that hydration administration cannot be billed if other drug administration codes are being billed for the encounter. As well as her recommendation to read the CPT preambles and instructions for better understanding.
After reading that information, perhaps post an actual scenario and we can help you determine the appropriate codes. We can discuss and discuss drug administration, usually though, I've found that it becomes much clearer when coding actual cases.
Kris