Andre LaRenzie, Chapter 11, Question 6

In this chapter, I found one specific topic very astonishing. Chapter 11 was all about International economics. The author, Charles Wheelan, speaks on the topic of  "evaluating official exchange rates relative to what PPP would predict".  He uses the term of burgernomics to describe this. In 2010, McDonald's Big Mac costed, on average in the US,  3.57$, and 12.5 renminbi in China. That would put the exchange rate for dollars to renminbi at around 3.5. The PPP prediction actually had the Reminbi at around 1 US dollar to 6.83 Reminbi, which is significantly higher. Later on in the passage, Wheelan states that China endorses a policy on "cheap" currency, and that this causes tension between China and the US.  This is where I find it astonishing and amazing and am left with questions. Can China really set an exchange rate like that? And does someone benefit and lose from this situation? Mcdonalds? The US economy? It doesn't sound right, and I can see why this increases tension between the two countries.