http://www.moneytalksnews.com/why-youre-probably-better-investing-than-donald-trump/
Forbes actually starts over 30 years ago when Trump claimed to be worth $500 million and he would be worth $20 billion if he had simply invested in the S&P 500:
The math
Imagine Trump had retired in 1982, sold his real estate holdings and invested his $500 million in the S&P 500 — that is, 500 stocks representing the American stock market.
From 1982 through the end of 2014, the S&P 500 index had an annualized return, including reinvested dividends, of 11.86 percent, according to
MoneyChimp’s S&P 500 Compound Annual Growth Rate calculator.
Per this calculator, every dollar invested in January 1982 would have been worth $40 by December of 2014. That means Trump’s initial $500 million would have grown to $20 billion. That’s twice what Trump says he’s worth today.