i think the answer is d
20 hours
i solved it this way : at 5 pm, the concentration of the drug is half the original concentration, which means that the half life of this drug is from 1 pm to 5 pm which is 4 hours ... a 240 ng/ml needs 5 half lives to get below 10 ( 7.5 ng/dl exactly ) and this would be 5 half lives times 4 hours each = 20 hours ... i hope this solution will be correct and useful
20 hours i solved it this way : at 5 pm, the concentration of the drug is half the original concentration, which means that the half life of this drug is from 1 pm to 5 pm which is 4 hours ... a 240 ng/ml needs 5 half lives to get below 10 ( 7.5 ng/dl exactly ) and this would be 5 half lives times 4 hours each = 20 hours ... i hope this solution will be correct and useful
