The formula D= 5 e-0.4h can be used to fid the number of milligrams D of a certain drug that is in a patient\'s bloodstream h hours after the drig was administered. when the number of milligrams reaches 4, the drug is to be administered again. what is the time between injection? Solution D = 5e^(-0.4h) 4 = 5e(-0.4h) 4/5 = e^(-0.4h) 0.8 = e^(-0.4h) ln(0.8) = -0.4h h = -ln(0.8) / 0.4 h = 0.5578 hours.