A certain drug is administered once a day. The concentration of the drug in the patient's bloodstream increases rapidly at first, but each successive dose has less effect than the preceding one. The total amount of the drug (in mg) in the bloodstream after the first dose is 42 mg. The amount after the second dose is 10.5 mg. The amount after the third dose is 2.625 mg. Write an explicit formula to represent this scenario. If the answer has 42 then it has a number and ^n-1 and if it's 30 then just regular + #.