The equation I used for these are: If N=Noekt then DN/Dt=N0kekt so the two problems I have trouble with is A radioactive substance has a decay constant (k) of -.0539 per year. If 371 grams of the material is initially present, what is the instaneous rate of change of the substance at times t=4 weeks and 18 months? so what I did is 371(-.0539)e(-.0539)(4/48)=-19.907 gram/week and 371(-.0539)e(-.0539)(1.5)=-28.421 gram/month Is this right? 2. A radioactive substance has a half life of 23.7 days. If 4983 grams of the material is initially present, what is the instantaeous rate of change of the substance at times t=1 day I find the k constant by -2491.5/23.7 which is -105.127 gram/day which mean -.288 gram/year? (by dividing 365). Is this right? then for 1 day 4983(-.288)e^(-.288)(1/365)=-2.948 gram/day? Is this right?