For a drug to be effective in treating an illness, its levels in the blood stream must be maintained for a period of time. One way to measure the level of a drug in the body is to measure its rate of appearance in the urine. The rate of excretion of penicillin is first order, with a half-life of about 30 minutes. If a person receives an injection of 25 mg of penicillin at t=0, how much penicillin remains in the body after 3 hours?