Explain how this adds up.

[Account Closed]

I'm reading " The Millionaire Real Estate Investor" by Gary Keller. On page 147-148 he gives an example of equity growth. In the last paragraph of his example he comes to the conclusion that equity in the amount of $128,506 was accumulated in 15 years. I am definitely NOT disputing his numbers, I just can't figure out the math he used to get there. Could someone who understands this please explain it to me? Thanks.