The rate at which a quantity of a radioactive substance decays is proportional to the quantity of the substance. the constant of proportionality λ is called the decay constant or decay rate (λ, like most physical constants, is conventionally represented by a positive number. that's why we call it a decay rate. if a quantity were increasing, we would use the words "growth rate" and also represent that by a positive number). write the initial value problem that describes the amount n(t) of the radioactive substance remaining after time t, if the initial quantity n(0) is 6.