In many ways you can think of it as the opposite of exponential growth. The e-folding time t is the time it takes to grow by a factor e. Parameters negative in the case of exponential decay.
P t the amount of some quantity at time t.
If r 0 0 then the formula represents continuous growth. So now that we have computed the decay factor we get that the exponential decay formula is. R is the rate of growth. In geeky math terms e is defined to be that rate of growth if we continually compound 100 return on smaller and smaller time periods.