It's easier to think of growth factor at first: So your number is multiplied by $g$ at each step. After $z$ steps, it is multiplied by $g^z$, so you end up with an equation $$g^zx=y$$ to be solved for $g$. To do that, take logarithms and end up with a linear equation for $\log g$.
Afterwards, you convert your $g$ to a growth rate $g-1$. Multiply by 100 if you want it as a percentage.
In your example, the equation is $$g^3\cdot10=80,$$ i.e., $g^3=8$. You hardly need logs to do that one, but if you do, logarithms with base 2 can't be beat: $3\log_2 g=\log_28=3$, so $\log_2g=1$, and $g=2^1=2$.