1. a prescription for a suspension calls for a dose of 200mg to be given twice a day. if the suspension contains 300mg/5ml, how many ml are needed for one dose?

As the dose is 2 times a day then the first thing we should do is calculate the number of milligrams per day:
 
 Then, we can make the following rule of three:
 300 mg --------> 5 ml
 400 mg --------> x
 From here, we clear the value of x.
 We have then:
 
 
 6.67 ml are needed for one dose