i = 0.20
j = i/12 = 0.0166666...
r = 1+j = 1.0166666...
1) Considering the payments and their reinvestment.
You should be able to add those up, but you may wish to wait a moment.
2) Considering the base investment.
...etc. up to 12 uses of "*r". This simplifies nicely to...
What happens if you add the two?
HOWEVER! And this is a VERY IMPORTANT "however", this plan doesn't work!!! Your 12th payment of 100.00 doesn't actually exist. There is only 3.04 in the fund so you have the unexpected 1,222.436, rather than the expected 1,219.391
What do your sources claim to be the right answer? What are your methods?