Let $Q = f(t)=15 (0.94)^{t/2}$ be the number of grams of a radioactive substance remaining after $t$ years.

(a) What is the initial quantity of the radioactive substance?
intial quantity = (include units: units)

(b) What is the annual decay rate of the substance? decay rate = %

(c) Evaluate:
$f(7) =$ (include units: units)

(d) Find a formula for $f^{-1}(Q)$.
$f^{-1}(Q) =$

(e) Evaluate $f^{-1}(7)$:
$f^{-1}(7) =$ (include units)