3. A drug is used to treat coughs. After the drug is fully absorbed, the amount in the body decreases at a rate proportional to the amount in the body. The half-life of this drug is 4.5 hours, and the usual single dose is 10 mg. a. Write a differential equation for the quantity Q of the drug in the body at time t in hours since the drug is fully absorbed. b. Solve the differential equation. c. Use the half-life information to nd the constant of proportionality. d. How much of the 10 mg dose is still in the body after 12 hours