This is a decay problem,
The equation is written as start value x (1 - rate)^tike
Using the information given in the problem:
a) Total = 120(1-0.12)^Hours
b) Replace hours with 19 and solve:
Total = 120(1-0.12)^19
Total = 120(0.88)^19
Total = 10.57674 mg
Round the answer as needed.