Reading Brandon Turner - Rental Property Investing

[Gary Stark]

I am very new to RE investing and reading the above book.  His first example "Plan" is to buy a fourplex.  He assumes he puts down $16k and $4k for repairs.  Total investment is $20k.   He assume each unit produces $200 per month cash flow - $9600 per year.   My simple question.  How realistic is this?  A 50% annual rate of return.  A two year payback on a real estate investment.   I am new and eager to learn.  I understand it take effort to find great deals.  However, before i get too far in here.. i want to know what i am missing.  This seems like assumptions that are simply not feasible.  Please help me understand.  His entire math for this "Plan" only works due to the above dynamic.