Write the formula for radioactive decay. If a function f has exponential growth or exponential decay then that is taken to mean that f can be written in the form f t c e k t. The rate of decay is great at first.
Mathematically a function has exponential decay if it can be written in the form f x A e -kx f x Aekx.
Final amount remaining after the decay over a period of time. A pdf copy of the work can be found here. This is the form of the equation that is most commonly used to describe exponential decay. So r is 23.