Integration : coupled differential equations

example : decay model :       material A decays at the rate α proportional to the amount of material A to material B | material B decays at the rate β to material C |  material C is stable ( no decay) | model : { material A = - α * material A ; material B = α * material A - β * material B ; material C = β * material B ; α = ´0.3 ; β = 0.15 }