The first problem that is show is exponential decay. First you will need to create a table with hours and substance left. The next step is to find the trend by noting that we are left with a certain percentage of the substance. One you have the trend you will use this to calculate the amount of substance left in said hours, in this case 6 hours. The following example shows you how to solve exponential growth. It is essentially the same except for the fact that you will be increasing, so instead of multiplying by a number less than 1 you will have to multiply by a number greater than one. You still have to find the trend and then use that trend to solve for the given number of intervals, in this case 8 years.
Have an iPhone? Check out all 200+ new features coming in iOS 13.