The NBA salary cap is a very difficult thing to figure out. In fact, it's so complicated that teams cannot just go out and make trades whenever or wherever. A NBA GM has to have a degree in statistics and calculus in order to make the simplest of trades.
Fortunately, the good folks at ESPN have come up with The Trade Machine to help with all your NBA trading needs. Simply plug in the teams and the players to make a deal and the software will indicate whether or not the deal is in compliance with the NBA bylaws.
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