Believe it or not, this is the second version of Q is very similar to the first, even though it doesn't really look like it. We can represent the second version as a * (1 + r)^t
See the similarity between this and the first version? Thus, we can represent what is given in the problem as:
Q = a * b^t = a * (1 + r)^t
Thus, b = 1 + r. Since b = 0.8, we can find r.
0.8 = 1 + r
r = -0.2, or r = -20%.