This is an exponential growth problem of the form:
F=Ir^t we are told that the sample quadruples every day so
4=r^1 so
r=4 then the equation becomes:
F(t)=a4^t and we are told that the initial sample, a, is 20 so:
F(t)=20(4^t) and if we let t=days=d
F(d)=20(4^d)
