I know the formula: A(t)=A(o) 1/2^(t/p), but I don't know how to use it in the following question:

A certain radioactive material decays exponentially. The percent, P, of the

material left after t years is given by P(t)=100 (1.2)^-t

a. Determine the half-life of the substance.

b. How fast is the substance decaying at the point where the half-life is reached?