Staking 20 coins will never result in being awarded a 100 coin staking reward.  I have done a very rough estimate based upon the last 3 months of staking and it appears that I am getting a tad better than a 2% monthly return on my staked coins (36,000).  Since it has to come in 100 coin increments, 20 coins is mathematically never going to have any real chance of producing anything.

I have about 4000.00 US of TZC at this time.  I get about 85.00 US in POS rewards every 30 days.  If the same numbers applied to you, you would get about 5 cents per month.  Since a stake reward is about 200 times that amount, you could conceivably get a reward in about 11 years.  But, that's not really going to happen.