So, I am wanting to vary a parameter in an equation with respect to time. Vary mass flow [ m(t) ] for an exponential decay to half its original value in around 60 seconds. I know the regular decay equation where: m(t)=m0*exp(-At) but I want the value to approach a steady state at 60 seconds (i.e. I am decreasing my pump/mass-flow by one half over 60 seconds). I need an equation which will allow this to occur.