Is this the type of math you had to do? Drug Calculations Quiz Page
Study some by yourself and figure it out. If that link is representative of what you saw then you should be able to learn it. Maybe the tutor did teach you wrong, but assuming you know what the definitions of everything is (for instance, I had to find out what exactly "drop factor" was) then someone with a high school education really should be able to figure it out.
I don't know how you can challenge the test grade. If your answers were wrong then they were wrong, whether or not you got it wrong because your tutor taught you wrong. Your only chance would be that they graded the test incorrectly.
I don't know enough about the situation to say what will happen to you as I have no knowledge at all of nursing education. What does readmitted in the fall mean? Will you have to repeat all the classes you've taken thus far?