Givens:
- 20% annual growth
- 30k payments/year
FV of Annuity$=P[\frac{(1+r)^n-1}{r}]$
$P=$ Periodic payment
$r=$ Rate per period
$n=$ Number of periods
$5000 = 30\cdot((1+0.2)^n-1)/0.2$
https://www.wolframalpha.com/input?i=5000+%3D+30*%28%281%2B0.2%29%5En-1%29%2F0.2
$n = $ 19.4 years
Alternatively
=FV(20%,19.4,30000)